To cover a whole-plate/full-plate image size ( 6½" × 8½" 216 × 165mm) I believe I'm going to need a 10" lens, 270-300mm focal length. Perhaps a little less.
Have read that early portrait photographers like even longer lenses, but I doubt my field camera and Thorton Pickard shutter would take much more weight than that. Right now I'm working comfortably with an f/7ish aperture lens so that will probably be enough.
It's a rule of thumb and there are exceptions. The Rapid Rectilinear lens was introduced in 1866 by John Henry DAllmeyer, this is still the era of wet plat use, although the first dry plates were used in 1871 it was another 5 years before they became commercially available.
The Petzval designs that predated the Quick Rectilinear lenses had poor coverage, but some were quite fast f4 and f3, and even f1 admittedly a 4½ inch short focal length, which was special order, for photographing children. A well corrected Petzval like my 10" f4 Dallmeyer Quick Acting Portrait Petzval is designed for 5x4 and Cabinet size, which is typically 4¼ x 6½ inches / 108 by 165 mm, so it will cover half plate and probably be OK with 7x5. The image circle vignettes on my 10x8 camera showing it won't cover whole plate.
The Wray Rapid Rectilinear lens I've fitted to my Houghton Duchess is marked 8x5 it's actually a 10" lens, the two Whole Plate RR lenses I have are both 12".
By around 1890 newer optical glasses brought major improvements in lens design, and so my 9" f6 Dallmeyer Stigmatic coves 8 x 5 inches at f6 and 12 x 10 at f16. The 10.7 inch Eq. Focus covers Whole Plate at f6 and 15 x12 at f16, these are small light lenses. There are similar improved designs from other lens makers, and these were sold with field cameras from around 1891 with the introduction of the Zeiss Series I Anastigmat Protar. The Stigmatic was designed ton 1895 by Aldis,then working for and advising TR Dallmeyer on newer lens designs, to compete with the Protar.
What am I concluding, if you get a Rapid Rectilinear lens then 12" for Whole Plate, but there are other options that cameras like yours would have been sold with.
Ian