Probably 30cm +/- a factor of two. Not too small, but not so large that they make the whole scope hard to transport.
>3) Does anybody have any good ideas on attaching the side bearings to the
>tube;
The box seems to be the best way anyone as thought up.
>4) Is there an essential difference in using smooth laminate as opposed to
> the rough type (for the azimuth bearing)? Which is better for this size
>of scope?
Yes. It is the nature of the Teflon/laminate interface. It creates a chaotic pattern of slipping and sticking which (when used with Teflon) gives HIGHLY desirable state of the static (stiction) coefficient of friction nearly equal to the dynamic (sliding) coefficient of friction. Smooth surfaces are stickier. Also the coefficient of friction varies with speed of slipping, with faster slipping giving more damping.
>5) Would it be a good idea to put an additional teflon pad in the middle of
>the ground board (with the central bolt passing through it)?
It would make the scope turn easier, but it will be at some sacrifice in stability. With the kind of central bolt bearing typically used on Dobs the maximum stability occurs when all of the weight is transfered directly though the teflon pads (as far apart as possible) to the feet directly below the pads - no flexing or twisting forces in the ground board. If the central pad takes weight it will cause the ground board to flex. This flexture will vary somewhat as you are turning the scope (applying force) which will make the movement "springy" instead of the desired "stiff" feeling. The BKD, Bill Kelly Device, does the same thing with a spring loaded plate on the central bolt. A teflon pad at the center will also slip very slowly compared to the others. The faster slipping at the points of the largest triangle you can manage is more desirable in terms of damping and "feel."
It might be acceptable if you have stability to spare.
>6) Are chair gliders as good as teflon (for this size of scope)?
No. They may be "good enough" for a lot of people, but they are not "as good" as Teflon, even for small scopes. I took the nylon pads off several small and medium sized scopes and replaced them with teflon which improved the feel on every one. These small telescopes were usable with nylon pads, but clearly it was not as good as with teflon. I have also seen several nylon pads, in different styles, which broke (cracked in half) in normal service as telescope bearings. Teflon won't crack.
Teflon should be loaded at 15psi/sq inch (to those in the rest of the world, I'm sorry about the goofy units) for optimum dynamics.
Low friction is not really the goal in telelscope bearing design. The goal is to get the right kind of friction. A sticky bearing will overshoot when you try to nudge the scope a little bit. A properly designed scope has wonderful "buttery smooth" feel. Once you use a scope like this, you will never be satisfied anything else.
You will probably have to play around trying different things to see what is good enough for your type of observing.
Chuck