Compressors can be useful for other things too. For example, if you ever flush out fork lowers with degreaser, it’s really useful to blast the gunk out with compressed air, and dry the insides.
I’d get one with at least a 25 litre reservoir. Mine has a 100 ltr tank, and is a bit bulky and takes about 5 minutes to fill up, but lasts ages.
I agree with Northwind – if the compressed air doesn’t cause the tyres to pop out onto the rim straight away, you’ve probably got other problems, i.e. rim strip too bulky for tight tyre bead etc.. However, I’ve often found that having a minute of continuous air while I manipulate the tyre bead into place usually does the trick, and hence the large reservoir has been a life saver for me, as I’m only running Stans on my bikes and don’t have dedicated tubeless rims/tyres.