dh and I are thinking about a conservatory but have to get the balance right as we have a small garden and want to leave some space for dc to play. The idea is to go across the back of the house (almost the full width of garden) and not too deep into the garden but with double doors/french doors that open out completely so on nice days dc can use conservatory more like part of the garden (a patio with a roof).
The size will be 4m across and 2.4m deep. I find these things hard to visualise and have no idea what sizes people usually go for. Will this be a useable space?
yes, but don't try to put much furniture in. Group your stuff at the ends so there is no obstruction to walking through. If you put garden furniture just outside the doors you can sit out there on a fine day. DC can have shelves, toybox, bench at one end. Big ride-on toys will cram it.
A round table wil be easier to sit round than a square one. Check that the chairs will push under it.