So how big of a MLT do I need to shoot for if I want to do 10 gal batches of all grain? I realized the grain bill has an impact, but is a 5gal cooler enough? Or should I just step up to the 10gal round cooler?
For 10 gallon batches, definitely 10 gallon cooler, especially if you are going to brew anything with a larger grain bill. Mashing 20 lbs of grain using 1.25 qt/lb requires 6.25 gallon of strike water without considering grain absorption.
If you are considering 10 gallon batches, certainly a 10 gallon, maybe even 12 if you think you might ramp up even more in the future. I have a 12 gallon rectangle because i already owned it. If I were buying new I'd go with a round one.