Hi Andy, one thing you will notice with a small lathe and that is that the chuck doesn't have much holding capacity except maybe for small round work.
The biggest drawback is also the spindle bore size, which in my humble opinion is the most important aspect of deciding on a lathe's capability. (been maching for more than 50 years)
Nowadays you can get fairly decent cost effective Chinese lathes that have spindle bore sizes that are quite large compared to lathes of 30 or 40 years ago.
If you go for a spindle bore size of 25mm then you can use 25mm diam material through the spindle, and all the rest of the lathe's proportions fall into place naturally.
It just depends on what type of work and size you intend to do.
I would suggest that you apply the bore size capacity to your needs and then see what lathes are available round that.