as far as i know, you only have a choice between two brands of slide toppers, Dometic and Carefree of Colorado. so that make it easy.
you can go to either's website and figure out what size you need, if you have the measurements of your slideout.
CW also has a webpage to do the same thing.
we ordered ours through CW, who also installed. it's been a great addition to our '07 Roo 23SS and has worked great for the past 8 years.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC