How to set key font size in gnuplot

I converted the comment of @Mareczek above to an answer, as I think that it should be an answer.

In case you have tics and/or labels at the bottom and left side of your multiplot-graphs only, explicitly setting the left- and bottom margin, will fix the issue of unevenly sized graphes, i.e.

set lmargin 10
set bmargin 3

You can play around a bit with the margin-values to see what works best for your terminal style.

To adjust the length of the samples:

set key samplen X

(default is 4)

To adjust the vertical spacing of the samples:

set key spacing X

(default is 1.25)

and (for completeness), to adjust the fontsize:

set key font "<face>,<size>"

(default depends on the terminal)

And of course, all these can be combined into one line:

set key samplen 2 spacing .5 font ",8"

Note that you can also change the position of the key using set key at <position> or any one of the pre-defined positions (which I'll just defer to help key at this point)