25 stone is nothing compared to some FAA PPL's i met in FL.
One guy used to fly a 150 he had to sit in the middle and I don't know what he did with the rudder pedals.
God knows what the m&B was like with full fuel and a hot humid day in FL.
Shouldn't be a problem with a 172 or possibly even a tommahawk depending on the the size of your arse (the trim wheel is in the middle next to the seat). I have had some heafty trial flights in and we have always managed. And with the ladys its mostly makes me blush. "Err I am just going to alter a control next to you I am not feeling you up" reply is usually along the lines " why not? feel all you like there is enough of it".
The joys of highland women
And with regard to the PPL medical if they are a bit funny about your weight there should be no problems with a NPPL. There are enough lorry drivers i have worked with with nicknames like sumo, jabba etc which make 25 stone look anorexic.
MJ ( who is no skinny either)