(TL;DR at bottom for people who would rather not read the explanations)
Before I start going through some tips on saving stash space, I can't emphasize enough how important it is to push to get your tasks completed. They will help you level up quickly which will give you access to the flea market. You will be able to sell most things at a significantly higher price than the trader price, buy items which will help you storing more items, and justify getting rid of the trash in your stash instead of holding onto it.
The tenets of stash management:
Start stacking your backpacks. You can do this infinitely with most bags, and this will allow you to collect many large backpacks without being in stash space poverty.
Take pistol grips and magazines off of your guns to gain a few extra slots of space in your stash. This excludes pistols and most SMGS. If you take them off of an AK, you can get yourself an extra slot or two depending on if you have a barrel attachment. On top of the free space, you also gain the ability to divide your guns into spaces that they otherwise wouldn't fit, such as singular columns/rows or inside bags and rigs. I also recommend selling magazines that you can buy from vendors, as they will needlessly take up space in your stash.
Wear your largest items on your PMC while you scav run. Lets say that you have a look at your stash and equip these items on your PMC to give you more space whilst scav running:
Your backpack stack (assuming your largest bag is a Berkut).
A Ratnik helmet.
A pair of goggles.
A pair of headsets.
A 4×3 armour like the 6B13s.
An Azimut rig.
2 AKs.
A pistol
Equipping all of these items will give you 72 more slots to store your scav loot in. You will never have to delete items from your scav run again doing this.
Rush getting your Scav Junkbox as soon as possible. You will feel a massive burden taken off of your shoulders as you gain rows on rows of free space. On top of that, consider grinding to get Magazine, Ammo, Pistol, Dogtag, and Med cases, a doc bag, and a wallet. These will free your stash even more. Thanks to these items, my standard stash can hold a ridiculous amount of items and still have room for multiple raids' worth of loot.
Use gas tubes to hold a ridiculous amount of attachments whilst only taking up one slot in your inventory. I use the 6P20 Sb.1-2 and put an X-47 handrail on it, but there could be more efficient combos (I believe a setup with the trax 1, 2, and bridge attachments maximises stash space the best, but they aren't easily accessible). By using this combo, I can hold a handguard, two scopes, a foregrip, three tactical items, and a mount for even more attachments. This only takes up one slot and didn't cost much to do.
Use bags and rigs to get the most spaces possible out of your stash. On these two pages you will see a column called 'efficiency', which is how many item slots you get for each slot taken in the stash by the bag/rig. Ideally, you want the highest efficiency possible.
The set up for a lvl 1 standard stash:
At the bottom of the stash (as to not contaminate long term items with fresh loot):
2 Pillbox bags filled with LBCR and CSA rigs to maximize the space available (pillboxes are the inexpensive option and are available from Ragman lvl 2 if you have unlocked it. Buy the CSA rigs from lvl 1 Ragman, buy the LBCR rigs from the flea).
An SFMP bag (a medical bag that takes 20 slots whilst providing 48) or a medcase, depending on your funds available. The medcase gives more slots whilst taking less, but is significantly more costly.
A magazine case, ammo case, docs case, dogtags case, pistol case, and a wallet (the pistol case is optional and not necessary unless you like hoarding pistols).
Two gas tubes to hold mods.
Any more things that will be staying in the stash for the long term (cowboy hats and ushankas for Dressed to Kill, for example).
At the top of the stash:
Loadouts. Guns, helmets, armour, headsets, face covers, backpacks, rigs, and anything else to be taken into raids.
Food and water for long raids such as Woods.
Nades.
A Weapon repair kit to keep good guns at 100% (not for use on scav guns).
By using this stash setup, you will have a space in the middle for loot which will be clearly separated from your loadouts and your long term stash.
In terms of how much space these items have saved me, I have 14 rows and 10 columns free for loadouts and loot (even more without a few misc. items and rigs floating around). I do not scrounge for inventory space anymore, and can comfortably have hundreds of barter items, meds, and plenty of loadouts without suffering from a lack of stash space.
TL;DR:
Stack your backpacks.
Take pistol grips and mags off of your guns.
Wear as many items as possible on your PMC before scav running so that you don’t run out of space when offloading.
Rush to get Scav Junkbox ASAP, and get the other cases too.
Use gas tubes to store many mods in a single slot.
Use bags and rigs to store items in as they can hold more than the space they take in the stash. Particular bags and rigs explained above.
