I have not done a lot of felting, mainly Scottish Balmoral, but this is my take.
How much does the yarn shrink when you felt it? Knit swatch, at least a 4 by 4 inch, take a piece of paper and trace around the un-felted swatch. Put a safety pin on the bottom front of the swatch, the swatch will shrink differently for the stitches (up and down) then the rows (side to side)
Felt the swatch and calculated the shrinkage length and width.
If the swatch shrinks 20% top to bottom, increase the number of rows by 20%, side to side by 15 % increase the Number of stitches by 15%.
Most important that this does not have to be exact, You can stop the felting once the piece is the correct size. If it is to large re-felt it.
Do you have a pair of shoe trees, preferably the wooden ones? The wooden ones have the front that is shaped like the front of the foot. When the slipper is about the right size use the shoe tree as a form for drying.