DVD rights to a 100 minute B&W film I directed years ago have just reverted to me.
I have a digi-beta of the film and want to redit it for streaming and DVD on FCP on an imac i7.
Should I have video lab turn the tape into PRO RES 422?
Would PRO RES HQ file be to big to handle?

ProResHQ isn't too big unless you have a very old system, which you don't. It might be overkill for SD DigiBeta but get it anyway. It will fit on a typical new 500gb hard drive (use a FireWire drive) easily, 250gb might be more than enough. An HD 720 movie file in standard ProRes is around 90gb for under two hours. It's worth the price to get a pro house to capture it as they should be using a good SDI to ProRes convertor to do it. Cheaper than renting the equipment and easier than setting it up.

PRO RES HQ is a very good choice for this as William suggests - Only better options quality wise would be 10bit Uncompressed - but the space requirements and hard drive speed requirements are going to mean you will have trouble playing that back efficiently on your average desktop system, and the quality difference isn't what anyone would call obvious.

Digibeta is 4:2:2 compressed at around 90 Mbps. You should be fine with prores 422 LT which is around 102 Mbps. If you want to be on the safe side, go with prores 422 but HQ is overkill. Especially since yours is B&W.

ProRes LT looks pretty bad. its noisy. I thought I would use it, but after some tests, it was just ugly from the very first clip. Standard ProRes works fine. Going to HQ nets you very little, but east 2X more drive space.

Using the AJA data storage app on my iPod indicates that 100 minutes of SD footage in ProRes HQ will occupy 54.8GB...

That's a no brainer for me. Just go with ProResHQ.

If you were talking about editing everything from scratch again and there was 60 hours of footage to ingest, I might advise differently but for 55GBs...