I have cut cheap bamboo shades, but it depends on the headrail and the mechanism that raises and lowers them.

Yes if the headrail is wood. I bought two that were too wide for our windows and cut both down to size. One I cut by hand using "dykes"--I don't recommend that method. The other my DH cut w/circular "chop" saw. We rolled it up really tight, made sure the edges on the non-cut side were even, and he sawed it. Turned out smooth and much more even than the one I cut by hand.

I removed the hardware on one side of the shade, rolled it up and marked where DH should cut, then reattached the hardware after the cutting was completed. DH drilled new pilot holes into the headrail for me to reattach the hardware.

The blinds I purchased were not only too wide for our windows, but also a couple inches too short. I decided this wouldn't matter b/c we'd rarely close them all the way (and we never have) but it's still kind of cheesy. Great way to save money tho!

I have cut matchstick bamboo rollup shades. I would think it's possible though. I did as melle did by wrapping them very tight only I then wrapped them twice around with painters tape on both ends and marked half of what I needed removed from each end for a balanced look to the threaded weave. Maybe I'm just obsessive, but I couldn't have done it any other way. I used our miter saw (or as Melle called it a chop saw) and carefully cut through it all, turning as I went to keep from having any fraying on the bottom side of the cut.
The tops and bottoms of the shade were made from larger hollow pieces of bamboo. If the header of your romans are metal they could be cut with a pair of tin snips, but it would be pretty much impossible to keep that out of the way of the miter saw. If you have a miter saw check to see what the blade/blades you have are meant to cut.

Thanks that is what I was thinking. The header is wood so that shouldn't be a problem. I was just going to cut off an inch on either side. Will have to try it and see what happens!

Just seemed silly to pay twice as much for a shade in the garage!

A few other possibilities--doing an outside mount, if that's not unfeasible or you don't hate the idea of it.
Also, when I was at Lowe's earlier this week, they had very reasonably- priced wood blinds that they would cut for you.

If you don't have a miter saw you can also use a hack saw with a thin blade. I rolled them tightly and put painters tape at the ends where I wanted to cut. They came out great. Much cheaper than custom.