CONGRATS on your new baby
!!! Sounds adorable!
Whether or not you neuter is totally up to you. But, yes, you'd have less a chance of marking if you do neuter. It should not change his personality, so I wouldn't worry in that regard at all. If anything, he'll be even sweeter bc he won't be going crazy as an un-neutered male who can sense a female in heat from great distances (don't ever let him run loose, or he'd be GONE as an un-neutered male...).
The surgery itself for neutering is super quick and minor and while they're a bit groggy that first night, they pretty much snap right back by the next morning...so don't trouble yourself too much about any pain. Plus, you are free to ask for pain management, which I always do.
Also, neutering does not affect how long their legs are...don't know where/how you heard that, lol
. His size is genetically determined. He will be mostly full grown at 5-6 mths...but will continue to fill in a bit.
Best age? Some say 4mths to avoid the start of marking...others do it at more like 6-ish months.