Hmm - we have a tiny garden and I bought our boys an 8' one for Christmas. Fortunately I got it from Argos and they changed it for a 10' one the next day. The 8' is OK for one child at a time (which is all they are supposed to have on them, and it would be fine for a 4 year old). I do know someone who has one, whose dc do use it.
However, the 10' one is great for two or three kids at a time, which our visitors all seem to expect, and our boys have just lived on it for the last 6 months - 30 or 40 minutes at a time. DS1 is 7 and he really enjoys it.
We thought that it would totally dominate our garden, but have wedged it into a corner, and it is not much more dominating that the 8' one was. The height of the poles make even the small ones fairly noticable.
Don't know about the concrete slabs - you would need to check with the suppliers - you might need some sort of matting underneath the legs just to stop it wobbling a bit. The enclosures are brilliant.
That 10' one you have linked to is an amazing price! Trying not to sound too biased, but we are very pleased with our 10' one, for two very bouncy boys.