I agree with AdamH. Basically, I'd suggest that if your current passion is for a 'big idea' then develop it into some sort of first draft. The process in itself will teach you things. But, don't plan to go all the way with this big idea immediately. Just work on it if that is where your current motivation lies. There's no point working on something you don't care about. If you can't find an angle that makes a 'smaller idea' more interesting to you, then go with whatever gives you the drive to write for the time being. Just don't put all your eggs in your best basket. Put the basket on the shelf for a little after you've got a draft done, and work on some other projects.
I have a TWO big ideas. Neither of them am I pursuing right now. Both are most likely going to happen next year at the earliest. I've developed both a fair bit, but I know they need a lot more work too. I think it would be getting ahead of myself to go straight for those BIG IDEAS first. Instead I decided to work on a stand alone story, in the super hero genre, in order to show I can write, I can add a fresh outlook on a caped character, and that I'm in the game when it comes to being a writer who has actually completed something... 22 full pages of self contained story. My medium sized goal with the stand alone project is for it to be a ticket to helping me in networking with the guys who are already in the biz, and making me an option for a back up story, or Marvel Adventures. Slim chance sure, but then I see my stand alone as a part of my networking tools to help promote who I am, and what I've done... as opposed to anything I am planning to pitch or turn into a series. I wouldn't say no to an offer, of course, but I look at it similar to the 'ladder' analogy that drgerb made above. Each rung... each step... needs to be small and upwards. Making a comic that you hope will get picked up right off the bat seems like trying to take a running leap at the ladder and hoping to skip the first few rungs. Why on Earth would anyone think they would be that lucky? Come on, work hard by taking the right steps and reap the rewards of that hard work.
This business is primarily about who you know. So you need to work on the networking as hard as you work on the writing.