For a petrol engined road car, you are typically looking at something that works well at about 3Krpm and pulls acceptably to 6K or so for overtaking.

what this means, is that strokes of around 60-90mm tend to work well, due the piston speeds involved.

Generally engines in those rev ranges tend to work well relatively close to square.

So you end up with cylinders of around 300-500cc being particuarly suitable.

So if you want a 1-2 litre engine, a 4 cylinder is a good way to go in that respect.

Then like you say its simple to package of course.