Welcome to the forum, Graham.
By size, do you mean scale?
I would suggest lower priced kits to "hone your skills." Learning to dry fit, fill seams, and solve problems are invaluable skills to possess. Find a subject that you like, read the techniques, tools & painting pages on the forum, and start. If you run into problems, the modelers here are more than willing to help you.
You could spend more money on kits that "fall together," which are good confidence-builders, but they don't give you much practice on the basic skills. This can end in an ability to build only those kits, which limits your choices of subjects. Its also a good idea to learn how to fill seams on an inexpensive kit, as opposed to learning on a $30 kit.
Hope this has been of help, and welcome to this wonderful hobby. Have fun !