What Size Board?

So I started surfing last August on the Florida East Coast. Took lesson from a small shop so I feel like my technique is pretty good. I'm riding a 9 ft longboard down line/ face pretty well on small and medium size days, but I want to get more familiar with shortboarding. So, I bought a 6'4 (2 5/8 thick) shortboard with what seemed like a ton of volume for me (I'm 5'7, 145 lbs), I can pop up and ride it when it's weak and mushy, but the problem I have is that I can't seem to paddle out back with it on the days when the surf is big enough (4+ @ 12+ secs) to use it in a way that's going to benefit me since it's impossible to duck dive efficiently for me. I'm thinking I need a much smaller board for bigger days, but I'm not sure how much smaller and what shape. So, what's a good size/volume for someone 145 lbs as an upper level beginner..?