I do wonder if fans proposing fanciful ideas of grounds of £10k or £18k capacity are getting carried away and have looked at the general standard of grounds in the Championship. Caldy play in a ground with a single main stand and then its very much a rail around the edge of the pitch. Nottingham, Rotherham, Jersey, Pirates are all much the same. Ealing has perhaps the best ground. Lets try walking before we can even jog. We don't have any money currently, have a debt to pay, don't have any players let alone be thinking several years on of needing a Prem standard ground. A non league standard footie ground with a ten year lease would be perfect for the new club and the championship. We were spoilt with the standard of facility at the CBSA.
I think you may be right, 18,000 is a lot for Premiership rugby standards, let alone rugby generally. The biggest stadium in the Premiership is 28,000 and that's Ashton Gate - which is also used for football. The next largest is the only non-football stadium and that's Welford Road. After that, it's a sharp drop down to just over 17,000 which is Brentford's ground (London Irish). After that, it generally sits around the 14,000 - 16,000 capacity stadiums that are for clubs which have rich historical roots to their area (Quins, Bath, Gloucester, Northampton etc).
Then you have Newcastle and Sale that are 10,000 and 12,000 respectively. This is where Wasps need to be at a minimum (subject to funding/ability to put a squad together) if they have hopes of rejoining the Premiership - provided the RFU/PRL don't pull the drawbridge up before then.
In my opinion, Wasps should only get to that point when/if a stable foundation has been made and they can seriously look to the future. Until then, we as fans need to appreciate (and I'm sure many do) that Wasps need to take what they can get that also meets the RFU's requirements and allows the club to grow sustainably.