I found this very frustrating as well, it isn't just Boden though, Yesterday was in M&S and they don't do age 6-7 in their school range, DS1 is about to turn 6 and is quite tall, so the 5-6 is too small, but the 7-8 is HUGE!
I always just get the size that they are if you know what I mean. So I just bought a 2-3 year old coat for a 3 year old and 5-6 trousers for a 6 year old. The sizes are generous IMO and I do seem to get two years out of them.
maybe will try 3-4 then and see - can you do free returns with boden or do you have to pay to send back?? ds is very skinny so needs the adjustable waist to be at its tightest on his current trousers to get the length. if i went up 2 sizes to 5-6, even with adjustable waist they would look like clown trousers
I find their sizing quite generous. DS2(5) is still drowning in some 3-4 Boden trousers that I bought for DS1. That's 4 years they've been going now.
Have you tried any French brands. La redoute, or that make they used to stock in Mothercare. I bought a few bits for my rather stocky (muscle - he has a six pack at 7) DS1 and they always seemed designed for tall, skinny kids.