Hi

As a beginner, can you recommend the adhesive thickness and grout line for a 20x25cm ceramic tile? I'm not sure of its depth and can't check right now but they are not thick! Can check tomorrow if needed.

Also, would a tubbed mix be suitable or should I get powdered? (I am in France and went to get powdered but couldn't see one that seemed suitable for shower walls).

Your grout line would depend wether you fixing to the walls or floor..

Powdered adhesive would be my choice.

As above, use powdered adhesive. My choice of joint width would be 3mm if it's walls and 3/4mm for floors.

Grout joint thickness is a personal thing although there are recommended minimums depending on what you are tiling. BS minimum is 2mm for walls and 3mm for floors. I try and always discuss joint thickness with my customers by giving various examples with the chosen tiles by laying them out with different joints widths. The choice of grout colour can have a bearing too as if a contrasting colour to the tile is selected a wider joint might be more appealing...if you get my drift??

All good advice above, with regards to the depth of tile adhesive, this will depend on the condition of the walls. I would prob use a 8mm square notched trowel (this is for powder adhesive, you shouldnt use a bigger trowel than a 6mm with tubbed).