I have a Bernina 800dl overlock, which may be the same model as what Morgan has. I can compare lots of other brands to my machine. I've obviously used and looked at too many sergers!
The B800 is considered a "basic" overlock, and it is what I would consider more or less "standard size". Specs: 40x37x34 cm. The trend, since I got my machine, has been towards larger, heavier, more powerful motors, and lots of fancy extras.
The Bernina 800dl is no longer in production, but you could probably find one used. The Bernina 1150, also no longer in production, is slightly larger (and more expensive at the time) but a much nicer machine. Berninas 1200 and 1300 are bigger. (and out of production?) The current Berninas L450 &-460 are hugh, massive. Take at least twice the space of my machine, if not more. If you own a Bernina 800 series sewing machine, one of these newer sergers and a coverstitch you also need a really big sewing space...like the size of a double garage would work nicely.
The Juki 635, 644, 645 are all pretty much the same as the Bernina800. Made in the same plant in Asia, most parts are interchangeable. (I ended up with the Bernina because I got a deal and it was much easier for me to get serviced.) If you really need a small overlocker, the way to go is to find a Juki model that is older than this series of machines. They will all be smaller; some will be quite small. (This is based on size, not price, nor availability!) The older Jukis were made for semi-professional use, in alteration shops and dry cleaners. They are good solid, somewhat ugly machines, with NO frills. They were made for abuse and hard work, like old Singers, so you can't kill them. Check that you can cope with the threading and lighting, as there is no hand holding built into these machines.
Babylocks are all much larger than my machine. Nicer, too!
Janome ...I don't know much about, but they are basically same as Elna, and same or slightly larger than my machine.
Huskylocks and Singer, slightly larger.
Brother might have a few slightly smaller machines, and lots of folks like them, or like the price. I don't like the position of the control levers and knobs, but Bernina does and made their newest models to match Brother layout.
All modern overlockers at the entry level price range will all be about the same size. The only really small overlockers that I've ever seen are the older semi-pro Jukis.