I can't tell you about Franke specifically (I hate SS sinks), but can offer my advice on prep sink size in general. Most of the time, we recommend a minimum 18" size sink here. Because I was using silgranit, I had to choose between prep sinks either 20" wide or 15.5" wide interior measurements. I chose the smaller 15.5" sink to save prep space on my island and am happy with it. I would NOT go any smaller back to front or in width. I do sometimes wish for slightly wider.

As a side note, you're on the right track picking your sinks first before ordering yourself into a corner with sink base cab sizes.