I just placed an order for a 24v trolling motor.  Last year I purchased a new AGM battery for my old 12v trolling motor, so it only has one season on it.  Would it be a problem to use a new battery with the one year old battery?   What about using two batteries of different size?  I'm thinking both of these solutions would be less than perfect as the "lesser" or older battery will draw from the other bringing its performance down.

My current battery is a size 30 and takes up all the space in my battery compartment, so I'll probably end up replacing it with two size 24 batteries anyways.  

I do not think it would be a problem to get a new battery with your current one. I do not think one year would hurt. 4 or 5 might make a difference. I would make sure they are the same size and type. Do not mix AGM with flooded cell etc... They have different discharge and charge rates.

Size, capacity, matters. In a 24V system, 2 batteries in series, your total capacity is that of the smallest battery.