Big & bold, not over cluttered, and what you
do is more important than
who you are.
Drive around and look at other vans, many times you are there wondering what the hell it is they do, their name is up there as bold as brass, but you are scanning all over the van to find out whether they are a plumber or gas fitter, a decorator or a builder.
I have my number up there big and bold, but I don't think I have had a single phone call as a result of it being on the van....
However; That signwriting has pulled in no end of work, one guy followed me all the way home (why he didn't just ring the number that was in front of his face for 3 or 4 miles beats me though).
I've been approached in the street so many times now, ditto while parked up munching on a pastie (not the same pastie I might add

) or putting fuel in the van at a petrol station, parked up at the local Tesco's and so on and so on.
Potential customers need to know what you do at a glance, often they come and ask you on impulse, if they have to spend ages trying to work out what you do, that moment may pass and they might not approach you...
Ian