Yeah, I won't deny it's hard to find a small idea you care about; there's a reason just telling us 'do a small comic first' doesn't turn us all into functioning creators overnight even though it gets repeated so often XD
The best advice I've got is these:
(ideas for shorter stories that you ... might have a higher chance of caring about than some random unrelated idea)
This might not be very helpful if the only reason you care about your long story is because it's a Big Long Epic Saga and you always thought it'll be super cool to create a Big Long Epic Saga (which is valid, that's how I started off when I first tried to make a webcomic :P) - but if you love your long story for other reasons as well, it makes sense to make a short story that shares those same traits you love 
(other short comics to reference if you're struggling to fit stories into very few pages)
Sounds like you'd want to write a long novel, and get it popular enough so that other people are willing to help you adapt it into a long comic :]