I started bass bone in my early 50s. I personally would recommend something "not huge". I found I'm a lot more effective on bass if I use a smaller bore slide, like a 547/562. I'd probably also avoid axial valves. Most of the best options are no longer made.
Your big struggle right away is probably going to be the air and the weight. You can work up to the air over the course of a couple of years, but there's no sense in making it harder on yourself. The weight will only become a bigger obstacle as you continue. My main bass is a Kanstul 1662i (which is no longer made, and was the lightest new bass I could find), but I also pieced together an Olds S-20 with double valves which allows me more flexibility and a lot more air control because it has the smaller dual bore slide.
Almost all the new basses made today start out big and get bigger. Nobody really caters to the "small bass" minority. You can find some odd exceptions, like a certain Amati
http://www.amati.cz/en/brasswind-instru ... 38-asl-382
Some older double valve models to look into are the rare-ish Olds P-24g (one of my favorite instruments), Olds S-23, King Duo Gravis 6b, and maybe 7b.
Some people will recommend a 72h for you, which is ok, but it's just a starting point. There are some other single valve instruments which kind of balance between tenor and bass, like the Holton 159, and several Olds models. There's also a Wessex Super Tenor, which is just one valve short of being a really killer small bass
https://wessex-tubas.com/collections/tr ... ier-pbf555
If you want to play bass, you'll 80% sure want a double valve. Some people make single valves work, but they are more tedious and you have to be creative. You can turn a single into a double with a plug-in valve, but that'll cost ~$1500 to have one made. You could get an Eb slide which technically gives you access to all the notes with a single valve (low C and B), but you'd have to use all 7 positions, and lose some convenient notes like C, B, F and E.
There's always the extreme solution
A used horn will probably cost ~$1500 up. New horns can be cheap if you go chinese (Wessex or Mack recommended options), or for a new Conn, Bach will be ~$4000+ or Shires, Edwards, M&W, Greenhoe, can be $6000+
Right now I'm struggling with a pinched nerve that effects my left arm, so instrument weight is also a concern. If you can afford it, a carbon fiber bass cuts a lot of weight
http://butlertrombones.com/?page_id=765
