If you are travelling, typically you will find 80"s available. I have done a live aboard that had a few 100s available for those that thought they would need/want the extra air. Also, in many destinations it is possible that a guide can lead a dive and when you are low on air have you and your buddy surface to be picked up by a boat while the rest of a group continues the dive. Coz is an obvious example where this happens. In these cases you are only pulling one other diver out of the water - if you are worried about it, be sure you tell your buddy - if he/she has a problem, find another buddy - I'll dive with a new diver - and I know many others that will.
As a "large beginning diver" don't worry too much about it. "Large" doesn't always mean faster air consumption, and "beginner" is a temporary state that we have all gone through. I'm a "small" diver and when I started I sucked through a tank faster than most. Take your time in the water, relax and try to do a little as possible. Just enjoy the dive and air consumption will improve with time in the water.