Find out the circumference of your preferred slide finger!
Fingers and Many slides come in varying sizes! If a slide is too big, you can put some gaff tape or moleskin inside.
If it is too small, bummer.
I LOVE the Dunlop 284 Eric Sardinias radiused brass and use it on my pinky. They come in other sizes.
https://www.jimdunlop.com/eric-sardi...-medium-slide/
Cheaper online….
Fortunately slides are relatively inexpensive. Sure easier than changing strings, too!
Have fun experimenting
Paul