I did not tried slit method, but i can give you an idea on size reduction:
Assume your patch size is W x H (width and height).
1) Try to move feeding line from center of patch to the left by distance approximate = W/4
2) Center frequency now be different. Do the simulation, find center frequency.
3) Reduce patch height H until center frequency is right
4) Tune W, H values to obtain desired bandwidth, etc.. Also you can tune feeding line position
5) Now you have antenna patch with reduced height H and maybe reduced width W.
I do not have any reference, i have seen such approach in practical patch arrays
If you still want to use slit approach, i would suggest to:
1) remove vertical slits and make more tiny horizontal slits (non-radiating edges)
or
2) make 1,2,3 or 4 slits at corners of patch, as in this article:
p.s. Ar you using sonnet? Your images looks similar to sonnet layout. If it is, your "box" looks a little too small for patch simulation.