Your first flat figure will be a straight forward, not to complicated figure
I would start with a 54mm and to start with just blocking the colours then go to shading and highlights.
the detail comes next, don't worry about cast shadows they come later remember this is your first flat figure and your finding new techniques, get as much advice from your club mates and it will happen
First figure I would recommend ? the BFFS Charles and Cromwell, not to difficult and can be painted in a variety of colours