I've mixed partial bags of developer from powder. You have to have an accurate scale, and you have to mix the powders up really well before measuring first, to get an even distribution of all of the chemicals. Also remember, than once you open up the bag of powder, you're exposing it to air that can still oxidize it, so you have to reseal it and get as much air out of the bag as you can. A decent digital kitchen scale should work (though make sure that you don't use it for food).
I use soda bottles to store my developer and fixer in. I clean them out really well (of course), and then squeeze the bottles before resealing the lids to keep all of the air out. By the time I'm nearing the end, the bottles get pretty flat, and I throw them away after they're empty. I use D76 and can usually get about 6 months out of a mixed liquid using that method, without losing any potency. After that, the developer weakens, so don't mix up more than about 4 months worth of that stuff (cause once it starts to go, it goes pretty quickly). As long as you keep it in powdered form, and away from air, it seems to last a lot longer. I have no idea how long the powder will last, as it's never gone bad on me.
Since I have to go through the trouble of heating up the water to mix it, and let it settle for a day or so before using it, I wouldn't ever consider mixing up roll's worth at a time. So I typically try to do 3 months worth at a time, that way I'm not mixing it all the time, and not having it go bad.