As long as the spacebar is positioned where the thumb you use it for falls naturally, it doesn't matter how large it is. I use right thumb on a normal QWERTY board and I suspect a 2x key would be large enough for me (since I tend to float my hands around a lot and don't touch type), and 1.5x for many touch typers.
I really like to use the thumbs for more, though (they are after all the most dextrous and strongest "fingers"), and love having thumb clusters on my DIY ergo board. I use a 1x key for space, but also use my right thumb for Enter, Ctrl and Alt. Left handles Shift, Layer, Backspace and Tab.
The JP variant of HHKB Pro appeals to me, partly because of the small space and positioning of the other keys, but I haven't researched it enough to know if I can remap the other modifiers to do what I want (and I'm sold on vertical column boards, so not sure I want another horizontally staggered one).