Apart from “how small” which has already been asked, the other vital question is “what sort of work”.
I bought a Unimat 3 when they first came out, which would be something like 27 years back. I now have some much bigger machinery, especially in the shaping and milling line, but the little Unimat still gets to do some of the things that are hard to do on a bigger lathe. At the time, I was a little torn between the Unimat 3 and the Compact 5, from the same stable. The pros and cons were that with the Unimat I could get more accessories for the money, and of course that the Compact 5 was bigger and did screwcutting properly. Probably the decider was that the 3 came with a free set of 10V castings. You can make a 10V with a Unimat, provided you have the milling attachment too. If I could go back and have a word with myself, I would say forget the free castings, get the biggest machine you can reasonably accomodate.
Comparing those two, from the same brand, the Unimat is a handy little machine, but better suited to say a boat modeller who wants to make a few fittings along the way. (Which is what I was at the time.) The bigger machine would be a better choice if you were looking at more of a Model Engineering type of use. Which is not to say that the Unimat cannot do a fine job within its size limits, for example that 10V, and John Wilding has made some good clocks with a Unimat 3. Either of those machines lends itself quite well to use in a confined space. You could make a kind of cupboard that opened up and had the machine inside for instance…I have seen a picture of that somewhere once. When I went to Germany to work for a two year contract I took the Unimat and a small Chinese bench drill, and set them up in the Keller (Small basement storage area. ) Borrowed some power from a convenient outlet, and got quite a bit done on the valve gear for a double ten. I bought a small cheap belt and disc sander to allow me to sharpen tools. It all fitted on a bench about 18 inches by 3 feet, and I had a small vice too. If you ever go to the Sinsheim show, or the Cologne one, you will see a lot of good work that has been done by guys in tiny workshops like the one I set up.
By the way, the Unimat 4 is essentially the same machine as the Unimat 3, it is no bigger anyway.
There are of course other good small machines on the market. Sherline for one. Anything that has been sold for a long period is almost certainly OK .
If you are setting up in a confined space, remember you do need to be able to sharpen tools, and depending on the projects, may also need to do the odd bit of heating, eg hard soldering and perhaps hardening home made cutters. They can be done out of doors perhaps, weather permitting.
regards
John