How to measure?

I was woundering if anyone could tell me the best way to measure a girth and saddle? Im getting my belgian this thursday and so im going to be buying a saddle but im worried I wont get the measuring right. Oh yea he is barley going on 6 years old so im told he will still grow some more. Is there a good way other than to just buy one and see if it fits? I have never had to buy a saddle for anysize horse so im a bit worried. Thanks

You can make a tracing for his withers and bring that with you when you go saddle shopping, so you can find a good fit for him.
Wither Tracings - How to Make a Wither Tracing for Saddle Fitting
( you can use a wire hanger straightened out)

Here are some websites on how to measure for a girth
Girths & Cinches | Information
How to Measure Girth Size | eHow.com
(that one is for a western cinch, I couldn't find anything on an english girth)

You will need a BIG saddle, lol. He is quite wide! I would go with full qh bars, with as wide and flat of a gullet as you can find. For the seat size, I would guess at a 17". You look about the same size as me and that's what size western saddle I take. You'll need to have the saddle before you can get an accurate girth measurement.

Abetta makes a good wide tree saddle that won't break the bank. They're made in the USA on a quality Ralide tree. I have one of their Endurance saddles and it's held up very well. The equisuede seat is just grippy enough without being TOO sticky, lol.
Abetta Cordura Round Skirt Saddle - Wide Tree: Saddles Tack Horse Supplies - ChickSaddlery.com
Abetta Pathfinder Trail Saddle – Wide Tree: Saddles Tack Horse Supplies - ChickSaddlery.com
Abetta Cordura All-Around Draft Horse Saddle - Draft tree, this web site can help you with wither tracings to determine the best tree size for your new boy.

If you want leather, Simco's supreme Draft is a nice saddle and comes in a 17".
Supreme Draft Horse Special Saddle by Simco: Saddles Tack Horse Supplies - ChickSaddlery.com

Dakota makes a nice Draft saddle too.
15" 16" 17" Dakota Draft Horse Trail Saddle 214

I found out hard way that there is no chance for me to guess correct girth size for english saddle. Somehow western never was a problem, but I used 46 on english wintec and now have either 48 or 50 on collegiate. Looks like it depends on saddle a lot. I basically just bought it from local store with "return" policy, so I was able to exchange 2 of them before I sticked with one, which fits my horse.