You have to use a text object if you want to change the font size of the tick labels.
Example:
x = 1:10;
y = rand(1, 10);
figure(1)
plot(x, y)
xt = get(gca, 'XTick');
set(gca, 'XTickLabel','')
xts = regexp(sprintf('%.1f ', xt), ' ','split');
fntsz = 18;
text(xt, -(1/(10*fntsz))*diff(ylim)*ones(size(xt)), xts(1:end-1), 'FontSize', fntsz, 'HorizontalAlignment','center', 'VerticalAlignment','top')
I made this as adaptable as I could, but you will probably have to experiment with it to get the result you want.
EDIT — Added separate font size variable ‘fntsz’, so the text position automatically spaces correctly w.r.t. x-axis.
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