As your libraries swell in size, the amount of data versus the available space in iPod and iPhones can cause conflicts. That is, 2 gallons won't fit into a 1 gallon bucket.
What would be nice is, when you are building your playlist, that there is an indicator of how much space the playlist is currently at. That way you can keep going with your playlist until your desired space has been met.
Cheers,
Fleetz