It really depends on what sort of finish you want but you
can get electric cylinder mowers which will take away some of the effort of a manual or petrol mower.
Take a look at the GoY garden centre (tab above)

60ft X 60ft a fair sized lawn. you may be fit now........ a cylinder mower will give a good finish and are available petrol driven and easier on the old body

recently bought a flymo strimmer at b&q, it is top quality so look at them

If you really mean 20m square and not 20 square metres I would forget a push mower. Petrol - well depends on how tall and how strong you are. I'm five foot two and always found starting a petrol one very difficult indeed, until I started standing on a slope with it below me, which compensated! I'd be tempted to go with the battery electric, unless you are happy handling a long cable.The cable electric ones are a lot lighter to handle - its swings and roundabouts.

Why not consider one of those solar powered robot mowers? Just leave them to run around your lawn and nibble away at the grass on its own.

A friend of mine has one and they're great to watch. We sat in his garden last year drinking wine just watching it roam around the lawn all day cutting the grass perfectly. They are fantastic, and cost nothing to run.

Can cost a fortune in wine though, Myron :-))))))

Thanks for comments think i will go for electric cylinder or may be flymo those robot ones sound good never heard of them savana120