Is there a MB limit relative to sequence listings?

Yes, 100 MB. When attaching a sequence listing, the TEXT file must be uploaded separately from other files.

If a sequence listing TEXT file is larger than 100MB, applicant must submit the sequence listing on CD via the US Postal Service in accordance with 37 CFR 1.52(e) and 37 CFR 1.821.

USPTO FAQ - Questions and Answers (18AUG2008)