For example, 3x6 tile on a shower wall is time consuming to install verses 12x12, larger is faster and more room to manipulate. But one thing in common they have is if that wall isn't level the joints wont be even. Joints being where one wall meets the next wall. So, if the wall isn't level you'll have a curve. So prep work is the most important part rite from the get go.
A few other key point I would like to add about importance of stone work are listed below from experience and taught knowledge.
-Safety: Don't stab yourself with a box cutter, wear safety glass's when cutting, fear power tools(you will respect them more), wear mask when needed, knee pad's and kneeling pads are your friend long term, good rubber gloves are a hand reptile saver. Muriatic acid cleans thinset well(extreme warning when using, see labels on bottle).
-Sticking something to a wall: Wiping it with a damp sponge(helps it stick).
-Clean: Tools, will last longer and work better if treated well.
-What works: There are many tools that do many different jobs but there are tools that work for an unintended purpose. Research and learn tips and tricks you can use because there are many. Most of the time if your stuck the web can help.
Overall try your best and never stop learning. This type of work can be hard but rewarding. Be a pro for your clients!