Don't make it too cluttered, keep it simple, people need to know in an instant what it is you do, and what you are is a WINDOW CLEANER.
Then they need to get your contact number at a glance too.
Those are the two main things that need to stand out and grab you.
Your company name needs to be there of course, but don't put too much info on there, it detracts from the main message.
The size of the lettering should be - biggest = what you do and your phone number.
next = perhaps your company name and perhaps how you work - Water Fed Pole, or ladderless window cleaning or (as in my case) Pure water window cleaning or any other idea that you think makes what you have on your van unique.
Drive around and look at other vans, and see how many are going around that takes you ages to work out whether they are a plumber, a sparky or any other trade.
Some can look dead smart and fancy, but by the time you've worked out what it is they do you're passed them.
Take your time Malc, don't rush into it to quickly!
Ian