Hi Gerald
I think I saw a pic of you once and you're 3 of me. I'm so slim mate, I have to run around in the shower just to get wet

If I were you I would consider:
Bungalows are good to wfp, they are quick with far less strain on your body not having to extend a pole.
Smaller houses - 2 or 3 bed semis rather than mansions. I think the profit margins on smaller houses are better and you can have a break between each one. Also you won't get stressed if one mucks you about.
Do a cheap leaflet and be proud to be a 'one man band'. I advertise myself as the small, friendly, caring window cleaner and I pick loads of work off the bigger companies that 'splash and dash'.
Pick an hourly rate that you would like to earn, say £25 per hour. Think about how long the job will take, if you think it's half an hour then round it up to £13 maybe £14.
Never go in cheap...do the opposite.
Tell the custy that you do a top job, clean all the frames and sills, front door, back door. Hand wipe all the downstairs sills. Wipe over the front door so it's not dripping wet when they open it. Be confident and polite. People buy from people and if they like you, you're in.
You're a big guy so have some banter and make them smile. I'm as skinny as a rake and I tell them 'i'm built for speed'.
As for marketing yourself I quite like the idea of 'opposites' eg 'The Tiny Window Cleaning Company' or 'Slim's Window Cleaning'
Health wise (without being rude) how about 'Slimming World'? My missus started a while back (yes she's considerably bigger than me) but it's great. She can eat as much she wants. And it works. There is a guy at her club who started in February and he's just lost 5 stone. Big celebrations. Just a thought but you might want to check it out.
Good luck to you anyway.
Hope it goes ok.
Cheers
Robbo