The usual answer is that it depends on two things: 1) your budget, and 2) what you want to do with it. (And in this case, I’d say the usual answer is valid.)
I would say don’t get anything smaller than a 20-ton press. That’s the size that’s most common anyway, so not a hard thing to do.
If you want a pressure gauge, find out if there is one, or can you add one to the jack (not only if it is possible, but is it something you, personally, can do, want to do, or know someone who can do it for you).
I don’t have a gauge on mine (a medium PotterUSA) but I don’t do a lot of production work so I’m fine with learning and paying attention to the ‘feel’ of the pressure. A gauge is very helpful if you are planning to do a lot of the same thing, or want to do some serious testing and need the specifics a gauge can give you (that last is a reason a gauge is on my want list).
There’s also the clearance issue. How deep of a clearance will you need to make the things you want to make? My medium PotterUSA is fine for matrix dies (aka Silhouette dies), pancake dies (aka blanking dies), and other things that aren’t particularly deep.
(Remember to measure the depth of whatever pushers you may be using in addition to the dies, etc., themselves and urethane.)
This was a pricey lesson for me: I bought a beautiful set of Durston mushroom formers for use in a hydraulic press and, you guessed it, they are too big to use in my press. So measure.
The top brands (imo) are Bonny Doon and Durston, with PotterUSA fulfilling a lot of the less-expensive but highly popular market. Mine’s a medium size and has been (mostly – see above) just fine for what I do.
They all offer different sizes, so make sure the one you get will do what you want to do now, and possibly in the future.
(And if you really want to get into using a hydraulic press, be sure to get Susan Kingsley’s book, AND download the Urethane handout that’s on Cynthia Eid’s website under the drop-down Handouts tab at the top.)
You can also build one, but that’s not my thing, so others will have to chime in on that.