The Rule book mentions 4'x4' table as a starting point, yet the weapon ranges are fairly-to-very large. What size tables are people using? I'm using 15mm on 6'x4'.

I haven't played enough yet to really see how this pans out, but I'm guessing that exchanges at Long range are largely non-events and Short/Medium (which have slightly more reasonable ranges) get messy depending on Quality?

I realise that weapon ranges are large in life, but not sure how they fit here (not that I want the other extreme of a certain Global companies view that weapons in the distant future only go 2 feet ). Doubly so when you consider the movement rates, units seem like they can close to Short range quite quickly and the variation in fire is lessened?

Use lots of cover

"medium" range is standard firefight range. Once you close the range, casualties do increase markedly (the range penalty for medium range tends to mean one less hit per attack) while long range fire is mostly just harassing / suppressing fire. Little actual effect, especially against armoured targets, but it'll cause them to keep their heads down at least.

My current table is 4 x 6 and plays okay but you do want to have a bit more terrain on there than for, say, 40K

Troop quality does make a big impact. Your average conscript with an assault rifle has a short range of 15. His elite space marine comrade has a short range of 25, and a country yokel who just picked up a gun gets a short range of 10. So there's a huge impact based on who is actually shooting.

My table is merely 50cm by 100cm (circa 2' by 3'), but I use 15mm minis so I use a 1cm=1m scale (instead of 1in=1m) in order to make things more compact.

My old table was about 3 x 3. It worked out fine, but you do want a more dense table with lots of concealment and cover, to avoid units being obliterated as soon they take fire

I find that urban setups work great for small tables. The battlefield feels "zoomed in" to the important areas of the battle.