The 16" Finworks (40.64 Cm) will certainly be alot better than the 29 cm.
Which FinWorks design is the fin you are looking at?
I'm not sure how tall you are or how long your legs are but beach starting a 52 cm fin can be difficult as it's hard to get your foot up that high.
I'm sure with a little more practice, you'll be beach starting the stock 52 cm fin with no problems.
I'd guess alot of the problem has more to do with learning a new technique than it does actually getting your foot up onto the board.
If you are slogging upwind (rather than planing upwind "on the fin") then the size of the fin really won't make much difference because you are using the shape in the bottom of your board (the rocker) to get upwind, not the fin.
When you are plainng all the time, hooked in to your harness all the time and in both footstraps, get back with me here and I will explain how to sail upwind on "fin lift" and you will not need to worry about "getting downwinded anymore.
Why are you only slogging?
What's your windspeed and what's your sail size?
Hope this helps,