Find the point of contact
Before searching for another pair, look at where your longest toe meets the slipper. Does it touch the end while your heel is settled in place? Does the upper press down from above? Or does your foot move toward the front as you walk? These are different shopping problems. A longer sole may help with the first, but it tells you little about the other two.
Ask for the space you can actually use
Outside sole length includes material your foot may never rest on. Raised rims, thick front seams, and lining can all sit inside that outline. Ask the seller for usable inside length in your intended size, and whether the measurement accounts for the lining. If they only provide a size chart, establish whether it describes foot length or the slipper itself. Do not subtract a guessed lining allowance from an outside measurement.
Compare two shapes before two sizes
An open-toe slide removes a closed front, but your toes still need to stay on its footbed. A closed slipper with a less tapered outline may offer space in a different place. Put overhead photos side by side and check where the front starts to narrow. Use the pictures to decide what to ask, not to calculate millimeters from a perspective photograph.
Make the heel part of the decision
If you try the next size, check both slippers with your usual socks. Stand, then walk a few steps indoors if the seller permits it. Note whether you reposition the heel or grip with your toes to keep the pair on. A useful comparison note is specific: the longer size cleared the front but moved forward with each step. That tells you to compare strap placement or another shape next, instead of repeating the same size change.