There are knee mills of roughly the footprint of the two you linked to. Old American mills you have the Clausing 8520 / 8530 (and Johansen mills which the Clausing was based on), Rockwell 21-100 and midway between these and a Bridgeport are the Millrite mills. There are quite a few others in this size but very small numbers and I can't keep track of them.
With older imports there were quite a few made in Taiwan and China in the 80s and 90s of similar size under a slew of brand names. Some like Evan's Enco look liked scaled down Bridgeports, others are close copies of the Clausing and Rockwell mills.
There are several current import small knee mills sold by Grizzly and others, so I'm sure one of the Canadian importers probably has something similar. They are much larger than a mini-mill but much smaller than they look in photos.
Harbor Freight 6x26
Grizzly 6x26
Grizzly 8x30
Grizzly a different 8x30
In regards to space, unlike a lathe which is pretty well self contained within its footprint with most controls at the front, you need to leave some working room around a mill to allow for table travel and ability to get at things like belts, head adjustment and hand wheels.
Taking my Clausing as an example, it is a 6x24" mill, which is pretty small for a knee mill. With a 24" table you would think you just need a space just a little more than 2 feet wide for it, but in actually it needs about 4 feet, 16" for travel side to side (8" to either side) plus enough room to operate the hand wheels at each end. It also needs about 4 feet front to back as well to account for the depth of the machine and to allow the head to be adjusted in and out.
Actual floor space required is only about 18x24 inches though, most of the extra room being at the table height and higher so you can have some stuff around the base. I have shelving behind my mill for metal storage because there is a lot of space behind the mill below the level of the head.
I don't have much room, so I actually have to move the table to one side or the other to access the sides or back of the mill, but this is also a consideration. The table moves, so you can get creative in placement to use your space with maximum efficiency.
A bench mill will be both similar and different from a knee mill in space needs. Being bench mounted you have more control over height, and of course you can have all kinds of storage below the mill.
Height is a space issue I think many overlook. An 8 foot ceiling will work for most but with heights of 7-8 feet for many Bridgeport styled mills, anything less than 8 feet (as you may find in a basement shop) can result in a severe restriction of options.