To me, a resume is tailored to specific jobs while a CV includes everything.
I think a long CV is fine as long as it is organized efficiently with bullets. I do think its OK to cut down long explanations of research. Unless you're applying to a joint phd program, just a sentence or two about each research experience is fine.
I think its easier for a LoR reader to scan thru a long CV instead of trying to make a detailed letter without details to go off of. Dont be afraid to highlight details that you feel are super important on your CV.
The biggest tip I've recieved so far from the schools I've talked to is dont think that just because your prof has written A LOT of letters in their lifetime doesnt mean they necessarily know what needs to be included in the letter. See if any of the schools to which you're applying have a evaluation form you can give your letter writers so they know what to write about. For example, I'm from Oklahoma and OSU-COM is my top choice. They have two dif forms, a pre-professional evaluation and a D.O. evaluation.
http://www.healthsciences.okstate.edu/student/forms/supp_eval.pdf
They suggest these not be used in place of a LoR (a LoR is much better and contains much more detail), but in addition or as simply a guideline for the letter writer. The admissions coordinator told me how suprised I'd be to find out how many LoR writers forget to mention an important trait such as character, judgement, reliability, or simply interest in medicine in general.
Best of luck requesting your letters! Dont forget it's OK to give your writers a specific deadline and email them occasionally to make sure they havent forgotten! (sometimes the writers procrastinate just like us!)