Will this look silly on my wall is a fair question, and there is a practical way to answer it.
It is a fair question and there is a practical answer, but there is a thing worth understanding first.
Small is not large, reduced
A 5 × 7 is not a 14 × 12 scaled down. It is a different painting with different rules, because the amount of detail the paper can hold changes with the size.
At that size I cannot paint every bar of a grille, so I paint four and let your eye build the rest. The result is looser and, if it works, livelier. There is nowhere to hide a bad mark, which sharpens the ones that stay.
At 14 × 12 I can follow the reflections across a bumper properly. The painting can afford passages that are quiet, because there is room for the eye to travel. A small painting cannot afford a quiet passage. It has no room to spare.
So the size you choose changes the character of what you get, not only how much wall it covers.
The measuring trick
Cut a piece of newspaper or a cereal box to the size you are considering, add a couple of inches all round for a frame, and stick it on the wall with masking tape. Leave it there for two days.
That is it. It sounds too simple to bother with and it settles the question better than any rule of thumb, because you will walk past it and know within a day whether it is too small.
Two days is usually enough to know.
Rough guidance, if you want numbers
Over a sofa or a bed, the picture wants to be somewhere around two thirds the width of the furniture. A 5 × 7 over a three-seater will look like an afterthought no matter how good it is.
Hang the centre of the picture at about eye level for a standing adult, which usually means lower than instinct tells you. The common mistake is hanging things too high.
In a narrow space, a hallway or the wall beside a door, small wins. You are always close to it, so it never has to work from a distance.
And a group of small paintings hung close together reads as one large object. Four of the little ones in a block is a real option if your wall is big and your budget is not.
The honest answer
If you tell me the wall, the furniture near it and roughly how far away people stand, I will tell you what I think will work, including when the answer is that nothing I currently have is the right size for it.
The small pieces are gathered here, and everything available is in the gallery. Before it goes up, the framing notes are worth two minutes.
— DawnE