|Deciding what to customize
can be difficult...but first and foremost, do what you enjoy!
If you like collecting regular sized action figures, perhaps you look at some of them and know you can improve on their accuracy. Or you need to fill out your dioramas with all those obscure background characters that haven't been produced yet.
Perhaps you want to create 12" dolls .... you're handy with a sewing needle (or know someone who is) and want to make some characters that were made as smaller figures, but have been ignored in the larger doll line.
Or maybe you want to make IG88's ship to hang from your ceiling, or have your very own Krayyt Dragon attacking poor Tatooine citizens. It's up to you. Many customizers specialize in one area; some try their hand at all forms. But each speciality has its own techniques, obstacles and skill requirements.
There are other factors that will influence your decision, however ...
Budget - making smaller figures will definitely cost less in supplies.
Availability of fodder - if you can find a lot of cheap figures to cut up, great! Even beat up vintage figures can be restored to better-than-original condition. Cheap 12" dolls and ships, however are harder to come by.
Reference materials - before you start a project, make sure you have AT LEAST two reference pictures. The more angles you have to refer to, the better. If you have to guess or use "artistic licence", you can be sure you'll find a new picture surface somewhere to make your interpretation completely wrong.
Skill level / Age - if you're young or don't have much experience in sculpting, then maybe simple action figure repaints or "head swaps" are for you. On the other end of the scale, if you want to express your artistic talents, you may be able to build your customs from scratch.
Space requirements - if you want to start building ships, you're going to need more space to work than if you do smaller 3 3/4" figures. Keep in mind the room you'll need to store your project during construction and all the materials, tools and reference pictures as well. If you're doing smaller customs, you can be as portable as keeping a fishing tackle box of parts and supplies that is transported to your work table when the itch to customize hits. Or you can set up a dedicated work table in a well-ventilated, baby-safe and pet-free area. Wherever you decide to work, make sure you have a good lighting source as well.
So, keeping these issues in mind, are you ready to customize?