Define a (xo,yo) co-ordinate point and using that (xo,yo) point, express the solt (having width W and Length L). By changing (xo,yo) point, you can adjust the position of the slot. On the other hand, by adjusting W and L, you can adjust the slot aperture area. Therefore, in HFSS< define FOUR new variables: xo,yo, W and L. Run parametric study using these FOUR variables with some educational guess, you can easily optimize the size and position of the slot. It is better to keep W and L fixed in the first trial, and optimize xo,yo.

Thanks a lot.