FWIW, when you're dealing with trees that size and in that condition, getting them on the ground safely doesn't really depend on whether you are running an 18" or a 24" bar, or have a 50cc or a 60cc saw. Bigger saw with a bigger bar will make it a little more convenient, but has almost no impact on safety.
As far as bars and chains go, 20" is the longest .325 bar you can get. If you want to go longer, you have to move up to a 3/8" bar and chain which is VERY easy to do. Bar + chain + $6 for a 3/8 rim and you are ready to go. Having said that, I would NOT put a 24" bar on a MS261 saw. Not sure the oiler will keep up and even if it does, its gong to be a dog.
You've already picked up another saw, but if you were wanting a step up from your MS261, I would have probably recommended going to a 70cc saw. It would be a true power house with a 24" bar, but also very capable with 28" or even a 32" bar which will handle a really big tree. a 28" bar is also handy for noodleing large firewood rounds in half so they are light enough to be picked up and put onto a splitter. I've done that with 18" bars and 50-60cc saws, but it kinda sucks. 28" and a 90cc is a piece of cake, but 70cc and the same bar would still be easy. I run a Stihl 026 and 036 quite a bit and the 62c 036 saws are my go to work horses. A lot of guys run 24" bars on them, and they run fine that way, but I run them with 18" .325 bars. 18" will make 90% of my cuts in one pass, and I'm not wasting torque and HP trying to pull an extra 6" of chain that I'm not actually using. I've also swapped my drive bearing and sprocket out so that I can rum a 9 pin rim which lets me convert my extra torque into more chain speed. For felling, and bucking firewood, I really like this setup. If I'm working on something that's 24" or more, I'll bump up to my big saw, and that does happen with some frequency so I'm glad that I have the option. Anything much under 24" gets dropped and bucked with an 18" bar though.
As far as your Echo goes, I'm not familiar with them. Your carb may be tuned a little rich which is probably a good thing for the first several tanks of fuel. After its broken in, you might look to see if there's a dual port muffler available for it. Cheap upgrade that will help the saw breath better and help you get the most out of it without actually putting any extra wear and tear on the machine.