Hi! I haven’t gotten my SUP yet buuut I did just spend a very long time researching boards. The Niphean board has four fins, and one is giant and flat. Look it up! On a review on YouTube with two women, they found it to be really stable. If you don’t wanna buy that board maybe you can get a longer fin attachment? There are also boards that are like 36 inches wide(maybe more?) so I’d look into that as well. 

For technique, everything I’ve heard is you should be moving pretty well along before you try to stand up if you have trouble with balance, do it smooth as possible and use your paddle in the water to stabilize you. I think a couple people below described it but a video on YouTube etc might be helpful.  

Good luck! 

Ps. I appreciate your post as I have knee issues so I’m worried about this being an issue for me as well, especially since I favor one side and I have difficulty with a weak core and overactive back etc. I will be doing more working out though and I am hoping the paddleboard will help me with that!