"Short-focus Newtonians require much larger secondary mirrors than long-focus models."
Sky & Telescope editor Gary Seronik's 6-inch f/9 planet-viewing Newtonian features a secondary mirror with a minor axis of only ¾ inch — illustrating that long-focus reflectors can be optimized to use very small diagonals. However, this small secondary trades edge-of-field illumination for minimal diffraction. A larger diagonal would be a better choice for general observing.
S&T / Gary Seronik
Telescope makers agonize over the size of their secondary mirrors the way golfers fret about choosing the right club for a given shot. But the optimal size for a particular telescope's secondary mirror hardly depends at all on the f/ratio of the telescope's primary mirror. For example, all other things being equal, the same 1-inch secondary mirror will serve well for 6-inch reflectors from f/4 to f/10, in all cases producing essentially the same edge-of-field illumination — one of the most important parameters to consider when you are selecting a secondary mirror. (An important clarification: this logic applies to telescopes used for eyeball observing, not astrophotography.)
This graph shows how one diagonal-mirror size works for telescopes with different f/ratios. This example shows illumination profiles for four 6-inch reflectors with 1-inch diagonals. Althought the fully illuminated field varies in size, the more important parameter, edge illumination, remains essentially unchanged.
S&T illustration
The point is illustrated here. While the fully illuminated field of the long-focus reflector is greater than that of the short-focus one, the illumination drops off much more suddenly in a long-focus telescope once you get past the fully illuminated zone (the zone within which one sees all the light gathered by the primary mirror). What does this mean for makers of long-focus Newtonians? Unless they are willing to lose a lot of light at the edge of the field of view, they probably are going to wind up choosing a secondary mirror about as big as they would on a much faster (lower f/ratio) instrument.
You can fine-tune the size of the secondary to suit a particular observing program. Planetary observers are fanatical about keeping the size of the secondary as small as possible to minimize image-harming diffraction effects, while variable-star observers want larger diagonals for larger fully illuminated fields (which ensure that the brightness of a variable can be reliably compared with that of any other star in the field). But it is a gross oversimplification to say that short-focus reflectors need big diagonals and long-focus reflectors need only small ones.
