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Slice,
This depends entirely on what you want to do with the scope. Most of the telescope making books and several websites go into the importance of the size of the Illuminated Field. I will assume you don't intend to pursue photography with the scope because it requires a larger FIF and defeats the purpose of the scope because the secondary size becomes too large.
An old rule of thumb for a general purpose telescope is that the size of the FIF in mm should be 50% of the focal length of your longest regular intended for use eyepiece. In other words if you plan to use a 30mm eyepiece as your normal low power eyepiece you should have a FIF of 15mm. If your longest focal length eyepiece normally used will be 25mm then you need a FIF of 12.5mm. These sizes can be varied, if you intend to use the scope solely for planetary use on a tracking mount with short focal length eyepieces then a FIF of 5mm is fine, as it allows you to reduce the size of the secondary to improve contrast while maintaining image brightness on axis. The off axis images however would vignette and you would notice a little dimming of the images with longer focal length eyepieces at the EOF.
Personally, I wouldn't care for a FIF of less than about 10mm. Kreige and Berry recommend a FIF of about 20mm on their larger Obsession style dobs.
Cs-John B
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