First off, I am guessing this is a 4x4 rancher and not a 2 wheel drive model , correct??
if so, you should NOT be running tires of different heights to the front and reatr, your asking for drive line problems and or damage!, they need to be of the same Diameter from and back for the 4x4 system to work properly, and not causing binding in things, taller tires turn less revolutions than do smaller one's
so your front and rear drive train will not be turning at same speed and will cause binding, and breaking of parts IF tires do NOT slip first!>
IMO< going to any tire taller than a 26 on a rancher is asking way too much from your rancher, there geared higher, and don't have the HP to handle large heavier tires,
so, I would not put 27 's under one odds are you will NOT be happy with the power the atv has afterwards, and might get stuck more often due to not having power to spin and self clean a tire!, plus you can damage drive train having heavier tires, more traction at time son them and, making motor work a LOT harder all the time to move them!
NOW< your FOREMAN, what model is this , a 300-400-450,500???
then what yr??
they made a bunch of Foreman's over the yrs, so NOT knowing what yr and model it is EXACTLY< its hard for us to give you correct info or help!
when you add larger tires, you alter stock gearing, when you think most honda 4x4 atv's have less than 30 HP to start with,, HOW Much HP do you want to loose and still be happy
do you FEEL: your current set up, it has too much HP??
Odds are NO< is the answer, so then it comes down to a PERSONAL answer on HOW much power YOUR willing to loose to have larger tires!
every inch of taller tire , gets you a 1/2 inch of ground clearance ONLY, so, how much power loss is a little more ground clearance worth too??
and again, these are personal questions ONLY you can answer, as what I feel is too much a power loss, maybe you'd be happy with, and vice versa!
but we still need to know more info on yr. and model of atv's here to give you the best help!
all honda atv's are NOT the same! even when there Foremans!, as they made many different versions and sizes of them!