The 6x7 glassless negative carrier for this series of LPL enlargers measures 54.97 mm x 68.02 mm. Part of the problem is that there are, to my knowledge, no standards for 6x4.5, 6x6, 6x7, and 6x9 negative formats. The sizes are approximations of the actual dimensions.
Each camera manufacturer decides what actual dimensions to use. The width, of 120 film is commonly 61mm but can be a little less; a few mm margins on each side doesn't get a negative width across the film much more than 55 to 56 mm. You could, if you wanted to, remove the masking blades and have them machined down to give the maximum opening the glass will allow. Personally, I don't think it is worth it. I don't know if even the camera manufacturers match the viewfinder image coverage to exactly match the negative coverage.