I have a WM18 because it's the biggest I could get into my workshop. Although it does all I need of it, it's not the best of all possible machines. For example, I'm 6' tall, which means the table is a little low (backache). If I lifted it, I'd have a longer stretch to reach the Z hand-wheel (tippy toes). Ergonomically, the WM18 is about as big as this style of mill will go. Others fit power assist, and it's a good idea.
It does fit in the space though! Headroom a small problem – it's 350mm less than my 2.3m ceiling, so I had to be a little careful about lights and the up and over door. With the head fully down, there's space for an engine crane to lift it on to the stand. Otherwise, I'd have needed more manpower! Once installed, side movement of the table is an issue: full travel including the hand wheels is 1.7m. Although unlikely to be moved full travel, the machine would take up a fair whack of 6' x 6' shed space, not leaving much room for anything else, like a bench! Do-able, but a bigger mill would be more challenging, and perhaps impracticable.
I'd really like a bigger mill too, but it's not going to happen.
Dave