I've been playing around with SD8 and VLAN Pooling, however only 'Tunnelled' mode seems to support pools. Is that the case or am I missing something? If I select 'Bridged' as the mode the 'Interface Type' becomes greyed out. Selecting 'Tunnel' mode gives the pooling options.
That is correct. VLAN pool in Bridge mode is not supported.
In which case what's the best method of keeping the collison domain small on a Meru network while ensuring devices can roam? VLAN Pooling looked like the perfect solution but running the AP's here in Tunnel mode isn't an option.
How are other people keeping the collision domains to a sensible size?
I have not tested this, but can you try to see if this helps, configure the ESS in Bridge mode, and set the AP VLAN Policy to RADIUS VLAN only option". Then on the RADIUS server configuration set the VLAN pool name.