Lets not get into what the size of a secondary should be for a particular design. Everybody has different desires depending upon what they are doing and how much shading that they will accept and other things. If the secondary isn't off by much from what you think that it should be then take it and run. What you got is probably for a bit further offset of the focal point and will be quite satisfactory, especially if you want to do some photography with the system at some future date. Have Fun and Good Luck with the primary mirror.