I thank peterrubensitein and weiss217 for taking the time to reply to my query. I have not yet invested in a vacuum sealer. I have relied on the manual pumps provided with the sous vide bags I have bought through Amazon. Perhaps I'll have to buy a sealer.
I followed the suggestion of someone who told me to buy two-gallon Zip-lock bags. I'll have to see whether I can fit a beef rib roast into one, and I'll also have to try using the "water displacement method" to remove the air from them and see whether I succeed. If I don't, I'll have no choice but to invest in a vacuum sealer.
The depth of the sous vide bags that I have was not a problem. It is the narrow openings at the top which gave me trouble. I managed to maneuver a two-rib roast into one but was certain I'd never be able to get a three-rib roast in.