My suggestion is to allow some sort of "set xlength" command (similarly for 'y' and posibly 'z') that would set explicitly the length of an axis (e.g. in cm). The canvas size should then be calculated accordingly to fit the whole figure, including axis labels.
This is badly needed since currently, with the "set term <type> size <X> <Y>" command, figures with varying label sizes (especially in the 'y' axis) produce varying axis lengths. Having figures with different axis lengths in the same manuscript is aesthetically unappealing.
Presently, the only solution I have been able to find is to change e.g. <Y> by trial an error until you get the desired length (or ratio of lengths between the x and y axis lengths).
PS: Thanks a lot for making gnuplot, it's such a nice tool!