I can only quote the Soil Association standards used in our guides
"If you are going to house your sheep, you will need adequate space – not less than 1.5 square metres per ewe. You will also need to provide adequate trough or hayrack space (not less than 50cm per ewe) and access to clean water. "
A ewe with one lamb needs 2m sq and with two lambs, 2.35m sq.
If the lambs are healthy and suckling well, and there is shelter in the field, they will be fine to go out after a day or two. But if you have the space and the weather's rubbish, keep them in. Since they won't be lambing over a long period (I assume) and there's only a small number, risks of infections aren't that high.
Good ventilation is essential though and keep them well bedded so that they are dry.