I found the default backpack space to be plentiful. If you're running out of space you have a looting disorder.
It's Borderlands....
The whole damn game is one big looting disorder <ahem> <grin>
I carry, on my siren -
Equipped :
1) Moxxi's Good touch (Fire SMG)
2) Vladof Swift Rifle (Purple Corrosive non-spinigun AR, because I'm working on the crouchy AR kills achievement atm, less than 200 to go for Rank 5, then I can never use an AR again...)
3) Moxxi's Bad Touch (Corrosive SMG)
4) Jakobs Chikamin Muckamuck (Purple Jakobs Sniper for one hit damage)
Shield - The Bee, usually. Saves me ammo, but makes me squishy if I don't see the mobs first. Got it after 3 tries at Hunter, I think I got lucky.
Grenade - Longbow Storm Front (I don't recall where it came from, but by god it hurts things LOTS, Orange Tesla grenade, on steroids)
Class Mod - Angelic Nurse (for HP regen and not having to worry too much about finding health, slow as hell regen though. Purple)
Relic - Vault Hunter's Relic (for the teeny tiny decrease to white item drop rates and the corresponding tinier increase to greens, blues etc.)
In my backpack I have :
A Level 47 Badaboom Bandit Rocket Launcher (the first Orange weapon I found, fires lots of rockets at once for one ammo consumed. this is a good thing, since the game's so damn stingy with Rocket ammo)
Storming Trance (Generic Maliwan Shock SMG, for shield stripping when required)
Conference Call (if you don't know what the conference call is, then you don't understand the rage at the 'bee nerf'. I don't like how it shoots, myself, but having a shotgun available means I can use shotgun ammo if I run out of other types)
Chulainn (Slag/Shock SMG, just got from PT2 Clan war, deciding on whether or not to keep it atm)
Rubi pistol, Corrosive, allows me to use pistol ammo, is a Binary, so 2 shots per trigger pull, and has the same damage to health effect of all Moxxi's weapons, so adds survivability.
Alkaline Turtle shield, in case I decide to try my hand against Master Gee. Probably going to kiss it goodbye, since I find the raid bosses to be massively unfun.
Adaptive shield, for when having 40%ish elemental resist would be a good thing.
Absorb Shield, becaues when it catches a bullet you take no damage.
Slayer of Terramorphous class mod, because hey, I had a Conference call and Bee before the nerf
For when using fire SMG heavily.
Witch class Mod, +corrosive mod, for use against Hyperion bots.
Cat Class mod, +SMG damage for when I feel like a change
Afterburner relic, for when there's a lot of travel to do.
Longbow Corrosive sprayer grenade, again for use agaisnt Hyperion.
So that's about 13/14 out of my 37 backpack slots used. 23/24 slots ain't a lot left over.
Oh, and NecrotechMaster, I've got the save editor, but that doesn't get used on the Siren, she's my first alt in BL2. Now my Gunzerker, after 190 hours on the Siren, I'd had enough. He got edited to hell and back. Bee, Sham, dual Infinity pistols, all weapons locked to my level as I played so I didn't have to replace things I liked etc. from level 10 or so. He was FAR more fun in PT2 than the siren because I just didn't *care* what I was facing. Shock/Fire or Shock/Corrosive unlimited firepower made things fall over.
I tried Gearbox's definition of fun, and didn't like it, so I made my Gunzerker to have fun my way