I’m in the same boat as you (2.5 year old BP around 650grams in a 24 x 12 x 12 pvc with 2 UTHs).
I just ordered an Animal Plastics T10 a couple weeks ago with a Pro Products RHP. I can’t remember the watts but I want to say it’s around 80 watts (maybe higher). I got this size mainly because it will line up with my existing T10 which houses my BOA. I would think a T3 would be fine for a male BP but I’m sure others can weigh in. As for humidity, these enclosures are great for maintaining adequate levels. I use Reptichip and a large bowl which does the trick. In the winter time, I’ll mist a little bit if I see the levels get too low.
The RHP should be sufficient but some people go with a UTH as well just for peace of mind. I’m only using an RHP.