We have many years experience of sizing wedding marquees and we like to ensure that there is enough room to accommodate all guests throughout the day/evening in case the weather is inclement and everyone is inside at all times.
Through discussions about guest numbers for both day and evening and an understanding of what the marquee is to be used for e.g. ceremony only; ceremony then drinks, dinner, dancing; dancing only, we can work out how much space will be required.
We would normally recommend that the dance area is kept clear throughout your event so that it can be used for the ceremony, or drinks reception, or buffet layout. Using the whole area for dining and clearing furniture for dinner afterwards can leave space very tight if the weather is particularly bad and may have an effect on the guests enjoyment of the wedding.