What size of cooler to use for mash tun?

I've brewed 6 extract beers so far, with only limited success. I want to improve on the flavor and complexity of the beer, and so I am considering making a mash tun with a cooler. If I do, I will start with a few partial mash recipes, but them probably move on to all-grain somewhere down the road. My question is what size to use for the mash tun. I see some people who partial mash use a 2-3 gallon cooler, yet for all grain all you need is a 5 gallon cooler if you are doing a 5 gallon recipe. So shouldn't I just make a MLT with a 5 gallon cooler, and that way it will allow me to upgrade to all grain someday, or am I missing something? I am only planning on batch sparging, but again I am a newbie to all grain equipment and procedure, so I'm not sure if that is the best to start off with. Also, if I did go to all grain someday, and had to boil 7 gallons of beer down to 5, could I use my 5 gallon pot and another 3 gallon pot, and just combine them into the carboy, or would I still have to get a 7-8 gallon pot for all grain? Sorry if this has been asked a zillion times, but I just realized this weekend how simple all grain brewing could actually be, and I might not have to get that much equipment for it.

I use a 5 and then a 3 if I have more grains than the 5 can hold. If I could do it all over, I would get a 10

10 for mash. For the boil, I wish I could do 10 gallons now to up my beer production without sacrificing too much time.

I have a 52 quart rectangular cooler. You will need 2 pots when you go to all grain so get a big pot for boiling ( 8 gallons or larger) and use the smaller one to heat water.

You need a MLT that is large enough to hold the grain, and a batch of sparge water.
I have a 5g cooler for a MLT, and I can get a 5.25g batch with a gravity <= 1.052 (possibly a bit more) while batch sparging. If I want a higher gravity, then I either have to switch to fly sparging (which doesn't need as much space for the sparge water), or make a smaller batch. With fly sparging, I can get a gravity up to 1.075 with a 5.25g batch.
I'm happy with this situation, but most AG brewers wouldn't be.

-a.

I have a 10 gallon round cooler, and I make 5 gallon batches. I thought about getting the 5 gallon cooler, but everyone here on the forum talked me out of it! I can make smaller beers, larger beers, do temperature steps via infusion or decoction, etc.

A 5 gallon cooler might be adequate for most beers. But what would you do if you wanted to make a beer that had 15 pounds of grain and called for a protein rest and a saccrification rest? You wouldn't have enough room in the cooler for that beer. That may be a stretch, but if you have a larger cooler, you won't have any concerns at all. I don't know of anyone who bought a larger cooler and regretted it.

Yeah, go for the 48qt cooler, there's no reason not to. Soon you'll be moving on to all grain, then, before you know it, you'll be wishing you could make 10 gal batches. Price difference is almost negligible if you go to Wal-Mart, and this way you won't have to worry about upgrading later and having an MLT you no longer need sitting on the side, taking up space.
As for the boilers, if you can, I would suggest getting a 7-8 gallon pot (or better yet, a 15!). Having a 5 and 3 really only gives you 6-(maybe) 7 gallons of boiling space (4-4.5 in the 5; and 2-2.5 in the 3), and that just might not be enough for some brews. I have a 30 qt and 14 qt, and I try not to put more than 20-22 qts in the larger, and 8-10 in the smaller. Works fine for me. Good luck!!

I would go for the 10 gallon round or square Ice cube cooler. You will never regret it. A 15 gallon pot is also a good choice and helps to prevent boilovers. You can make 5 to 12 gallon batches with ease. Pay the price and cry once. You will be happy you did.

If you are tight on cash, spend a bit more and get a 10 gal cooler and get a smaller pot. You can always use foam control drops to prevent a boilover (e.g., I do 6.5 boils easily in a 30 qt aluminum pot), but it is hard to 'make space' in a 5 gal cooler. You will want that extra space if you ever intend to brew a beer over 1.070 gravity, you have big sparges, or you need to do a multi-rest mash. And aside from the cost, there really is no significant disadvantage when using a 10 gal cooler instead of a smaller 5 gal cooler when batch sparging. Plus, with a 10 gal cooler you open the door to some 10 gal batches down the road, if you want.