There are a lot of different microlights out there, and empty weights and seat limits vary.
However, it's quite easy to check. If you go
here you'll find the list of microlight types approved for use in a flying school, and for each one a data sheet (TADS).
For example, go to the third page and look at the data sheet for the GT450 (a lovely aeroplane to fly BTW) - on page 1 we have:
Maximum take-off weight (MTOW): 450kg
Typical empty weight: 217kg
Limit per seat: 110kg
Maximum fuel consumption: 13kg per hour
So, a quick back of envelope sum tells us that two 17½ stone people, plus an hour's fuel can go flying together.
As for questions about weight and height - microlight instructors are very aware of this - it's an important issue to them. Just ask if at your height and weight they can teach you or not, nobody will be offended.
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