This is, believe it or not, a question we are asked quite a lot.
To be honest it is unlikely that you will be too heavy for your chosen ride-on, particularly if you are thinking of purchasing a lawn tractor or a heavy duty lawn tractor.
However, as a guideline some manufacturers recommend a minimum weight of 50kg (7.8st) and a maximum of 140kg (22st) for operator weights. This is partly to do with the sensor but also linked to the safe operation of the machines on slopes.
However, there are times when you could be too light. All ride-on mowers and tractors have built in sensors, under the seat, fitted for safety so the machine cannot be operated by accident when there is no-one on it or a small child sits on the seat.
It has been known for very light operators to think the ride-on does is not working when they first try to use it because they are not heavy enough to trip the sensor. Check your manual thoroughly before you assume this. Also, try checking the specs' of the machine before you buy it in order to work out if this could be an issue.