I'm inclined to think that this is not a subjective question. Of course, it might be considered so if framed in terms of "aesthetics" or "art". However the "art of typography" often comes down to some fairly matter of fact questions and answers, of which I think your overall question and how it might be considered is one.
Staying away from the subjective (although not necessarily from the complex or hard), I would frame and therefore break down your design decisions with respect to the psychology of your target group of readers.
The two principles I live by in this regard are these:
- Principle of least surprise
- Productivity principle
The principle of least surprise leads to simple but effective guidelines such as these:
a) adopt the primary, i.e., most familiar style employed by your readership's main monographs or periodicals;
b) settle on a single style in any single work, avoiding inconsistencies unless you consistently provide contextual cues that make it easy for your readers to (possibly subliminally) recognise your pattern in this.
The productivity principle is a little harder. It's all about keeping the rate of your median reader's information assimilation for expended effort as high as possible. Of course, this means finding the psychological sweet spot over many factors (see another answer regarding this sort of consideration here). Some of these factors are psychophysical, e.g., relating the motor performance of eye saccades to the optimal size of lines on the page, or to considerations like: if your readers are around professor level or higher, they are likely to be 40+ years old, hence likely to be presbyopic, and therefore might be better served by a larger font in text and diagrams. Other factors are cultural and therefore expectations based, e.g., having much to do with how your readers learnt to digest textual and graphical information earlier in their lives. Some factors relate to their subject competency levels -- low competency levels often suggest introducing more whitespace on the page, higher subject competency readers would probably prefer the page to be more information dense. Then there are cognitive factors that fit somewhere between the psychophysical and higher order factors, e.g., the commonly applied rule of thirds, the golden ratio principle and so forth.
Nevertheless, as a useful rule of thumb, I often (but not always) set figurewidth/textwidth == textwidth/pagewidth since that maintains consistent proportions across the x-axis of the page.
Regarding the reasonableness of spilling a figure into the margin: under conventional circumstances, I would tend to think not since it would interrupt the steady flow of the eye as it scans across the body text elsewhere on the page. Nevertheless, while remaining true to principles 1 and 2 above, Tufte-style documents are a clear exception to the
"don't spill figures into the margins" rule.
Summarising, while your question takes us to complex and hard places, "good" typography can find some serious psychological grounds. This, therefore ought legitimately be the stuff that TeX-typographers consider as part of their daily work. For me, unless the design is intended to push non-conventional boundaries, I always approach this matter by considering the psychological act of how the specifically-defined readership acquires textual and graphical information from the page. These considerations include weighing up psychophysical factors, and the target readership's hard and soft wired conventions, beliefs and expectations, etc. The beauty of this approach is that, apart from taking us from the realm of the subjective, it provides a framework to guide decision making about which rules to follow and how to break them when they don't apply.
If you want to be perfect, you use in all your graphics the same font (for example libertine) and font size (for example 12 pt) used in your document. So the best would be to prepare all graphics in a way, that they can included in your document without changing, scaling, etc. That has the advantage you can build all your images with the same resolution (for example 600 dpi).
If that is not possible, use the same kind of font (for example with or without serifes) and the same font size (I mean not the number here, I mean that it looks as tall/small as the other font).
If you have to change the fontsize, be not too small (everything, legend and numbers, must be readable!) and do not use a greater font size in your images.
For example with a fontsize of 12 pt in your document I would use as lower fontsize in the graphics 10 pt or 11 pt, not smaller.
If you have to scale your images prepare them (that could mean some trial and error runs ;-)) that they have an optimal font size for legend and numbers in the printed document.
Best Answer
The short answer is
The LM font sizes are optical sizes (looks best at that size). You should use the same size in Inkscape. Just beware that Latex point size is 1/72.27 inch and the point size the rest of the world use is 1/72 inch or the PostScript point size
The font size in figures are normally smaller but not less that 8pts
Yes you can use it if your main document font is LM roman
Since version 0.48 InkScape has the option to export the text and graphics separate so that the figure then can be processed with Latex to create a pdf. You can imbed Inkscape (SVG) figure in a latex document, see "How to include an SVG image in LATEX".
For free standing graphics the following is a Windows-DOS batch file to do the job. You can insert your local font setup in "texheader.tex"
Example