Hi, welcome!
I would definitely recommend having a look in either resale shops or of course in Hermes to help you determine the right size for you.
I'm only 5'1" tall, but a 25cm bag looks TINY on me--the 35 is the most popular size but as Maggiemoo said it can get heavy rather quickly. A 30cm is the only one I haven't actually tried to walk around with yet, and I think that one might be juuuuust right for me in terms of versatility.
A lot of the size preference has to do with your lifestyle, also. For instance, if you commute around a city and have to take a train or bus a lot, you probably carry a lot of things with you on a daily basis and would need a larger bag. But if you're like me and live a more suburban lifestyle where you drive most places or don't go a huge distance from home on a daily basis, you could get away with a smaller bag.
As for leathers, in my opinion Clemence is the heaviest of the more common leathers (I'm not counting things like Buffalo that are hard to find). Togo is a little lighter weight, and as you have already learned, not as slouchy/a little stiffer. Fjord leather is a wonderful alternative because it has beautiful character, lots of texture, takes color wonderfully and also has a lovely matte finish. It's also much lighter in the last year or two than it used to be--Fjord used to be considered a very heavy weight leather, but I recently weighed a Fjord bag against a Togo bag of the same size on a scale and they were within about an ounce of one another. Chevre is a great suggestion, but it's quite hard to find a 35cm Birkin in Chevre, at least at the moment. Something about small goats or some such
But if you do find chevre, it's a wonderful leather--lightweight, very sturdy, has a gorgeous sheen, and takes color very richly and beautifully. That also brings to mind the fact that I have seen a lot of HAC 32s available lately in Chevre; you might want to consider a 32cm HAC instead of a 35cm Birkin if you're unsure about size and think a 30cm Birkin is too small for you.
Other leathers not yet mentioned: Epsom which is least expensive; it has a very specific look to it and is quite thin yet very stiff. The one con to Epsom is that if your corners get badly scuffed or you get a bad scratch, it can't be refurbished as well as a lot of other leathers. There's also Swift, which is quite smooth and soft but even squishier than Clemence, and it doesn't sound like you want squishy? Then there's Box, which is shiny and the smoothest of all--but it can be quite high maintenance.
There are others, but these are the most commonly available at the moment so this should get you started.
Hope this is helpful!