what size trowel

Hi Ben

Since you're using large format tiles, you'll need to use a cement based 'bagged' adhesive. This will require you to use tile primer or SBR, don't use PVA. Also, depending on tile material, you may well need a flexible adhesive...check with your tile supplier. I personally wouldn't go with anything smaller than an 8mm notch trowel as long as the walls are nice and flat as you'll have problems with lippage if not.
One last thing to check is tile weight per square metre. Plaster skim has its limits to what it can carry per m2, and this also includes adhesive.

 

8mm minimum & 10mm trowel perferred.