Start with your heel in place
Put the slide on and settle your heel where it is intended to sit. Look at the longest toe without pushing your foot backward into an unnatural position. If your toe already reaches the front rim, usable length is the first question. If there is space while standing but your toes reach the edge after a few steps, also investigate how the strap holds your foot.
A raised edge is not extra footbed
Some slides have a visible rim around the sole. Include that rim when judging the look, but do not count it as flat space for the toes. Ask for the distance inside the rim in your intended size. A retailer may list an outsole measurement because it is easier to take. Confirm which measurement you are getting before comparing it with your foot.
Look at what the strap controls
A fixed strap gives you no closure to tighten if your foot moves forward. An adjustable upper may offer more control at the instep, though it cannot lengthen the footbed. Examine where the closure sits and how much overlap remains at the fit you need. Two bands can also sit differently from one broad band. More straps alone do not prove a more secure fit.
Avoid solving only the front
Sizing up can give the toes space while leaving too much length behind the heel or a looser upper. During a permitted indoor try-on, check where both ends of your foot sit after several ordinary steps. If you keep pulling your foot back, record that alongside your toe-room impression. For a new search, use terms describing the missing feature, such as an adjustable instep, then inspect the actual model. Do not assume that every result for long slides has a longer usable toe bed.