When I first became interested in this malarkey, my initial requirement was for some small 00 gauge wheels. My lack of knowledge led me to buy a Unimat1 toy machining centre.
It didn't take long to realise my mistake so I bought a s/h Hobbymat – 21/2 in centre height.
This machine worked, but as time went on (and not much time at that), and my experience grew, I discovered firstly that it still wasn't big enough, and second, that facing was convex rather than concave.
I then bought a new Warco 220 (rebadged Mashstroy C210T). With a centre height of 110mm and dbc of 500mm, it is just about big enough: there have been times when either a gap or a larger centre height would have been advantageous, but I have managed, in one instance by cutting the bar in two, machining, and then welding back together. In another instance by allowing the tailstock to slightly overhang the end of the bed.
In all of these, the weight has gone up – the 220 weighs 125kg when fully assembled, and this was just about handleable by me when I moved house 19 years ago.
The 220 isn't perfect, but I am managing to do what I want with it so I can't see me changing it anytime soon.
However, based on the above experiences, I do think that either a gap bed lathe or one with a centre height of at least 5"/125mm would be better. Lengthwise, I can't see much more than 500mm being necessary, and in terms of weight, well it all depends on your facilities, but for a singleton worker, probably a maximum of 150kgs. provided one can remove various parts to lighten the machine whilst installing.
Of course, if you know that right from the start you are only going to build clocks, or 00 gauge stuff, then a small lathe may well be satisfactory, but for general amateur engineering I think the largest you can install/afford is still the best idea.
I have to say that I do sometimes wonder if an additional tiny lathe might indeed have some advantages especially for the small jobs that sometimes have to be done.
Regards,
Peter G. Shaw