The size of the tic marks can be adjusted with set ticscale.
Syntax:
set ticscale {<major> {<minor>}}
show tics
If <minor> is not specified, it is 0.5*<major>. The default size is 1.0 for major tics and 0.5 for minor tics. Note that it is possible to have the tic marks pointing outward by specifying a negative size.