When I try to use slices, it won't me select custom sizes in both Hoz and Vert aspects. In the UI that asks for # of Slices or pixels, I can never set custom Horz / Vert pixels. If I pick 500 Horz and 500 Vert, it changes the vert without asking me, I can't ever get it to be square at the size I want. Is there a work around for this?
When dividing slices you are restricted to the size of the slice you are dividing.
In other words, you are cutting up a specifically sized area. The values in the Divide Slice dialog widow are not absolute values. If you enter 2 for horizontal, then the size is horizontally cut into 2 even pieces. The vertical value is automatically calculated based on Original slice size divided by number of horizontal divisions.
If you want exact slice sizes you need to not use the divide slice option.