Keep the original problem in view
Write down why you sized up. Was the old pair too short, too tapered, or too low above the toes? A larger size changes several dimensions at once. If length was already adequate, another front shape or a different width may be more useful to compare. Treat the loose heel as a second fit problem, not as the inevitable price of toe room.
Inspect the adjustment
A hook-and-loop strap can change the opening where it fastens. It cannot change every part of the slipper. Look at whether the closure crosses the instep, the forefoot, or the ankle, and whether the listing shows it open. Ask how much adjustment is available in your size. A decorative buckle or elastic insert is not necessarily a closure you can tighten yourself.
Choose heel coverage deliberately
An open-back scuff is convenient to step into but will not behave like a fully closed slipper. If keeping the heel enclosed is a priority, start by comparing closed-back styles. Check the opening and the heel material as well as the front. A stiff or bulky heel can introduce its own fit issue, so an enclosed back alone is not a guarantee that the pair will stay comfortable.
Use the socks you actually wear
Try both slippers with the socks you normally wear at home, following the seller's return rules. Notice heel lift, forward movement, and whether the upper needs repeated adjustment. Adding thicker socks or another insole takes up space elsewhere, including above the toes; it can undo the reason you sized up. Keep that tradeoff explicit. If one pair holds the heel but crowds the front and another clears the toes but slips, compare a third shape before deciding you must live with either problem.