ideal spacer size and tile on wood

[grace2much]

Hi all, I'm a plumber tiling my bathroom....nothing could go wrong.....he he!!

Anyhow, I'll be using tiles approx 10" x 18" with the 18" horizontal and not using a brick bond. I only realised that I can get 2,3,4mm spacers what should I use is there a standard size.

Also I need to tile on top of the internal mahogany cill board do I need special adhesive?

Is it ok to use the cheapest adhesive from say Screxfix or B&Q and not the big brand names?

I'll be tiling 13 sq mtrs so how much adhesive and grout will I need.

Many thanks for the help and your patience....