Hi Francesca,
the maximum length of busk will be the length of your front pattern pieces, minus your seam allowances at each end. You should be able to work this out from the pattern, but you may find it more helpful to sew a toile (mock-up) of the pattern with a solid front piece (cut the front piece on the fold) in order to check the fit. This will help you work out if you need to make any amendments to the pattern and whether you need to lengthen or shorten the front - which would change the size of busk you need.
For other patterns the length of the busk will depend on the length of the corset, which can vary between different styles. The width of the busk might also vary, and some corsetmakers prefer wider or thicker steel busks in order to achieve a more rigid front.