Second, look for reels of the approximate weight within your budget range that have the necessary capacity. Always remember that you're fishing for salmon, so you need a sensible amount of backing. The salmon doesn't know you're using a switch rod, so you're as likely to catch a big one as with any other rod, and you'd feel a complete chump to lose it for want of some backing.
Third, select the reel from that shortlist that most appeals to you. In the final analysis most people choose reels on aesthetics.
Guideline exceed #7 weight more of a traditional light weigh switch rod, I use this for sea trout and summer fishing it’s quite a soft rod with a shorter double handle. Reel is a LTS A 9-10 balance is perfect ,holds more than enough backing and the drag of great also has a nice sound as I hate loud clicks on evening sessions
I also have a Vision Onki which is a totally different rod a lot more powerful, stiffer with a longer double handle I use a Rovex waterborne #10 on this as I like the look of it and I like the drag system and they aren't very common (probably for a reason ) it balances perfect and again holds loads of backing if needed
I did have a Greys GX900 8/9/10 which again was a great size and balanced both rods well again with a good line capacity and drag performance
I must point out however that I’ve never really had my drag tested on anything bigger than a 10lb sea trout so if I have a good season there might be some gear for sale