You would export to full quality in whatever format it currently is (probably AIC). Then you would put this into Toast (with the Blu-ray plugin installed). You can adjust the encoding settings in Toast.
Bitrate affects how much data is used to construct video. DVDs are encoding in mpeg2 video, with a bitrate around 4.5 Mbps (megabits per second; take this and divide by 8 to get megabytes per second). Do the math, and you will come up with around 2 hours of 4.5Mbps fits on a 4.7GB DVD.
Blu-ray uses three possible video codecs: VC-1 (basically a new windows media), mpeg2 (like a DVD, but high def), and H.264 (also called AVC). Toast allows mpeg2 or H.264. H.264 is more compact but takes a LONG time to encode. A very general assessment of mpeg2 and H.264 is that both offer the same quality, but H.264 can use about half the bitrate of comparable mpeg2, at the same dimensions.
You have to decide how low you can go before the quality worsens. As I said, I would say in general you could set the bitrate as low as 11mbps for H.264. If there is a lot of action, or detailed textures such as moving grass or water, it might need to be higher.
I just opened Toast and starting dropping movies into an AVCHD disc. With the bitrate in encoding settings up to the max of 26mbps, I can put 40 minutes on. So you could play around with it, and see how your footage looks.
Clean, well-lit, tripod shot footage with noise filters and professional cleaning can compress very well. Professional BD sometimes go as low as 6mbps in scenes. Home movies have more noise and shake, and need more data to be compressed.
50 minutes fits, with a bitrate of 11.5mbps, with audio in 448kbps Dolby Digital.
