Hi Tom.
Will you just be storing vocalisations and using that as playback, or do you intend to record vocalisations too? This will limit you if you say yes to both, as MP3 players aren't to useful for recording (frequency levels no good to record). However, if it is just for a few calls to take out, then take your pick from the MP3 market. I guess people have preferences, but it shouldn't make a huge difference which one you go for, shop around and seach for the ideal one. Just a normal walkman type is fine, battery life will be good then.
Regarding storage size, check the file size of your recordings and work out a storage size from this. But get a much bigger one for future use I guess. Try to get one with a big LCD display, last thing you want is to be in the forest trying to read whats on the screen as opposed to keeping your eye on the bird. Try to get one thats backlit too, again, obvious reasons.
Like the others, give Wildsounds a call, they can be useful for information, but for just playing a few calls, not much advice is needed.