I think a panel like this is potentially a great idea. I'm always happy to see more in the way of cosplay panels that tackle interesting topics.
As someone who also gives cosplay help panels, I would agree that 2 hours is a bit long. Pare down the content a bit, since you aren't going to be able to keep people's attention for that long. I've given 2-hour panels, and while some people stayed for the whole panel, it was a lot of people moving in and out of the panel as it went, and there tends to be a smaller turnout than with 1-hour (or even 90-minute) panels, probably because people are intimidated by the length. If you wanted to give two 1-hour panels and felt you had enough content for that, you'd have to find a way to split it up so that each panel is unique somehow and now just a continuation of the other panel. I'd say a 2-hour panel is better than a 2-part panel, but that's also up to panels scheduling and what there is time for.
The other issue with this is determining how to teach these things -- teaching color choice can be done (in my panels where I talk about color choice it's only one or two slides on how to make colors cohesive, so going into how to make colors cohesive /and/ work with your skintone could easily expand into more slides, if doing a Powerpoint), and teaching proportion can be done to a point, but a lot of this type of thing, at least I've found, takes experience and a bit of intuition to know what looks right. I suppose teaching about general guidelines of proportion and body type would be good for people who are new, but knowing what looks good on your particular body seems to take a bit of an eye that isn't really teachable in the same way. It's not impossible, but you'd have to be really careful in how you actually go about teaching these concepts. I would focus on fit in this case.
I give panels on fabric choice, and I have to say that covering types of fabrics can easily eat up a TON of time that you maybe shouldn't use in a panel like this. If you cover what types of fabrics are used for particular garments, I'd try to keep it as brief as possible, and focus on how to choose fabrics based on a cartoony vs realistic look. I've given 2-hour panels on just fabric choice, so believe me when I say that it can take up a TON of time.
Honestly, I have a panel that covers a lot of similar ground (but more in a "here's things you may not think of that would really help take your cosplay to the next level" way), and it's my least favorite panel to give, so if this is something you are passionate about, please, go ahead and do it far better than I can! Though, if you want or need a co-panelist or any sort of input on structuring the panel or content or anything like that, let me know.