@sgrabenstein Thanks for reaching out, it sounds like a great trip you have coming up!
I will echo what @REI-LeslieN said about gauging the correct size for your compression sack: starting with the size of the stuff sack the sleeping bag came with. However, if your sleeping bags are, for example, synthetic and rectangular in shape, it could be very challenging to cut down on that bulk, even with a compression sack.
Most of the compression sacks we carry come in multiple sizes with the larger end of the spectrum being 20 Liters in volume with a couple at 30 Liters. For comparison, a daypack is typically between 18-25 liters in volume. If you think you'd be able to stuff your sleeping bag inside a daypack then you'd probably be okay. I also recommend sizing up on the compression sack if you're unsure between two sizes; you can always fill space with extra layers or other compressible items.
I would also recommend considering renting sleeping bags for your upcoming trip. It is a great way to get the gear you need without having to invest in a new sleeping bag right away. You can look at your local REI and see what equipment they have available to rent.
I hope this helps, have a great trip!