Measure your legs and put in its size (width and length).
Frame is the upper part that holds the leg inside. You can choose thickness and height of the frame.
Brim is the slightly wider part at the bottom (not the same brim that you define in your slicer). Put 0 (zero) to remove it.

Recommended to add 0.2-0.4 mm to leg width and length as it might be too tight (also our home printers are not very precise)

Total height of the item will be add_height + frame_height
Total width of the item will be
at the top: leg_width + frame_thickness 2
at the bottom: leg_width + frame_thickness 2 + brim_width 2
Total length of the item will be leg_length + frame_thickness 2