I tow heavy, I plow a LOT (most winters a 5+ mile trail system I try and keep open about 15 ft wide)
I plow dirt gravel, stone and you name it, I pull a Harrow, a disc, and a small like plow making food plots, I work the crap out of my 400
IMO< 25's is as tall as I would go
YES I felt a LITTLE loss going from 24 to 25's
but overall I am happy with the 25's, I would NOT go back to gain the small power I loss
25's are a nice sized tire , and way more options for you, main reason I went to them, were just too few decent tires to pic from in 24's to be honest for my liking!
Plowing snow is taxing to any atv, its mostly a first and second gear deal, so, some folks can live with more power loss than others, based on where and what all they plow
I plowed a drive way about a 1/2 mile long for 15+ yrs with mine on 25's, needed weight more times than I didn;t have enough power, but I still wouldn;t want to go taller than 25's on tires
like I first said, it all comes down to how hard you WORK the atv and what you can live with or not!
with 25's, if you DON"T lift, your fronts will rub the inner front fender liners, lifts are simple and cheap
OR you can do the lift thru shocks
you can move your ATV"s stock rear shocks up front, and then get 450 foreman FRONT shocks for the REAR of your 400
that is one way to lift the front end and BACK, to keep it level like, I just did a lift back in 1999, and ZERO issues from doing so!
another thing you can do besides JUST a lift kit, and I recommend this for you here, is you can BUY a set of front shocks off EBAY for about a 100 bucks, BRONCO's, they are adjustable and when I got mine(due to breaking a spring on my OEM's) they were about 2 inches longer than STOCK front shocks, they then lifted my front end(had to remove the lift however) and since your ATv is 20+ yrs old, new front shocks would be a nice help to handling for you any how, killing two birds with one stone so to speak?
without a lift again, or longer front shocks(same thing) 25 and taller tires will rub on you!