A short, if effective break down on Consumables.
Consumables, now have unlimited use per match for the same flat cost. so if you use a Large repair 5 times, you still only pay the 20k silver it takes to restore it after the match. Sadly, You still can't run 2 small kits side by side but a small and a large version can be done for the passive bonuses. Both small and Large versions have a 90 second cooldown.
For instance, in my T95 doom turtle I carry a small Medkit, a small repair, and a large repair. It rarely catches fire because of the angle on the back deck (its near auto bounce) so it's worth the risk to have the faster track repair and the ability to double untrack myself if someone gets cheeky with HE shells to my sides, or at the least rapid repairs to the tracks.
Bound to the 4/5/6 keys, these will save your butt.
The 4 slot should always be your Medkit. Why? your driver will be the critical (Cdr, gun, drv) crew member most often hurt. his position is the #4. so all you need to do when he's hurt is double tap 4 and its all good, and you never have to sweat out looking for buttons.
The 5 slot should always be your Repair kit. Why? your Tracks are going to be the thing you repair most, and it is the #5 button. So, just like the Medkit above, you double tap 5 and you are back in action.
The 6 slot should be your fire extinguisher. Why? dying to fire sucks is why. Some rebind this to the space bar or CAPS (or a spare mouse button) for faster reactions. pick a key that makes sense to you!
Now, the bigger question is Small, or large?
For pub matches, Small is generally the most cost effective. There are also advantages to the Larger consumables, ironically, you are also less likely to need them.
The interesting thing is that once these large consumables are used it doesn't give the full passive bonus until it cools down.
The Large Medkit gives a +15% bonus to crew survivabilty so it's less likely they will be hurt and you need it. Works great if you have a tank with fairly exposed crew members, such as an E25, or a low crew count such as the higher tier Swedes. Heals all crew at once instead of just the 1 that the small does. Remember that a crewman that is disabled at the end of a match DOES NOT get the full crew exp!
The Auto Fire Extinguisher (AFE) gives a 10% reduction for the chance of catching fire. (check your engine in 'research' for the chance of catching fire on a direct hit). It also has the benefit of only giving 1 tick of fire damage, instead of the minimum 6 ticks with the small manual version. These are the single most useful thing out there. All my tanks carry one and I rarely need it.
The Large Repair kit gives 10% (which is the same speed as 1 crewman (of 5) trained to 100% repairs) to the repair speed of all modules (tracks, gun) but generally don't bother unless you get tracked-and-racked at the same time a lot. (which is rare). Repairs all modules to full, instead just 1 for the small.
Food gives a direct and FLAT(additive) bonus to all your Major Qual skills AND to your crew's skills as well (Camo repairs etc). So your 100% loader becomes 110%! Gun handling is less Potato! (looking at you STA/B...) Some tanks need it to be comfortable, others it makes amazing.
All the gold shells and consumables can be bought for credits (make sure you have the silver option selected) or for gold (ok, but why?). Shell prices vary on the gun and tank, large consumables are 20,000 silver/50gold each. the small ones are 3,000 silver.
"But they are expensive and I'm broke!" It's better to have them on your tank and not use them, than to need them and not have them. A Dead tank costs more to repair than a damaged live one. If you think it's going to be a total loss then don't bother to use them and save the money. I can say I've never regretted carrying a full set of consumables, and in some cases I've used all 3 several times in a match and carried.
If you would rather buy when it's cheap, instead of Full Price, Nov-Dec they will usually go on sale for 50% for a short while. Generally around US Thanksgiving but there are other times of the year as well. Don't be afraid to blow 8-12 mil on consumables. I do. I've got 500 of all 3 Large consumables each and 200 of each nation's food. I don't sweat using them during the year. the smaller ones are super cheap and its easy to stockpile them for later use.
Also, and I know this is a thing, but please don't sell them off if you get them for free from an event or mission. I know it seems like free money and can help boost your bank account in the short term but all it does is rob your future self when you need another one. If you have a soft limit of a stockpile, (IE ~300) you can sell down to that soft limit (from 367 down to 300), but don't zero it out. people complain tier 10 is expensive for just this reason.