Notched trowel size for 15mm quartz flooring tiles

[Pigglet]

Hi, we've got some 600 x 900 x 15mm quartzite tiles to put down in a garden room extension we are currently finishing.
http://www.pricespaving.co.uk/product/silver-bullet/
The floor is a 7.5 meters by 5 meter 150mm thick concrete slab and contains wet underfloor heating. Tiles will be laid on flexible adhesive. Having never done anything like this I could do with an idea of what size notched trowel to buy? I was thinking of an extra large trowel with 10x10mm notches on trade tiler???
http://www.tradetiler.com/products/trow ... rowel.html
Would that be suitable?

Also, what spacing would be sensible?

[Colour Republic]

10mm trowel is the minimum you want, assuming the floor is pretty flat to start with and the tiles are calibrated. Double check that bit with the supplier as if not you may want a few different sizes.

[Pigglet]

Thanks for the reply CR, the tiles are calibrated but with your response in mind I might go for the 13mm notches then.
Can anyone advise on appropriate spacers given the size of the tile?

[Colour Republic]

If you're going for a modern look 3mm, if it's country farm rustic then 5mm+

If they are natural stone and not a porc copy, make sure you seal them well before grouting or you'll have a real problem cleaning the grout off the face of the tile