When choosing venues for the WC, what importance (if any) will pitch size have?
Will full-size rugby league venues get precedent over 'soccer-size' venues? Are there any size regulations? or is this impossible given using soccer venues in the UK is sometimes unavoidable?
Field size may well be an issue, field markings will be too. Even in existing RL stadiums field size isn't uniform. A number of traditional venues (e.g. Knowsley Road) had short fields and that really won't do for a few reasons.
First, playing on a shorter field affects how a game is played. A kickoff that travels 50 metres will land on the goal line of a proper size field, but would be close to going dead on the full at Knowsley Road or Wheldon Road. The field size needs to be uniform for
all top level games, with the possible exception of some slight leeway regarding width.
Second, it's vital that every game be played on a field having identical field markings just like every other major sport in the world. Field markings also need to be uniform for
all top level games. If the RFL can't deliver that they shouldn't be hosting a World Cup.
It can't be stressed enough how important it is for a World Cup to look good on TV, especially to outsiders seeing the game for the first time. I still remember the first time I saw the sport on video (it was a State of Origin game back in the mid '90s); I noticed immediately that the lines were numbered just like in the football I grew up watching. That, along with the other parallels to that sport, helped get me into RL. Having venues with different size fields and/or different field markings looks very Mickey Mouse compared to other sports that have World Cups.
They should have a field length component in the bid criteria for this reason, but I wouldn't be surprised if that never crossed their minds.