That's a tremendously good question. You can likely find some statistical market research regarding the size of the handbag market, but that will only tell you how many people buy handbags and who they are (demographics) which tells you quite a bit and is very important, but it doesn't indicate their willingness to buy YOUR designs (psychographics).
That will be tough to do without producing a few samples to shop around to consumers and/or potential retailers. Do you have some preliminary design specs that you can shop around?
If not, you can certainly indicate such in your business plan. It's okay if you don't have ALL the answers, and you can always update your plan as your business grows. But you'll earn respect from your audience if you acknowledge the aspects of your plan and/or business that need to be determined.