Have to buy a wetty this week, a 3/2. Trouble is I know that over autumn I will lose about 5kg. (I'm presently 80).
Would losing 5kg affect the fit and therefore effectiveness of a wetsuit much?
I prefer a wetsuit that is too large---as this effectively makes it much more flexy feel,they tend to last longer too,downside will not be quite so warm.
As long as you have good wrist and leg seals, the fit should not matter too much. Usually dudes lose their weight around the guts so a little flexability there won't matter because the harness will pocket the water around that area anyway. As long as the water inside your wetties isn't getting flushed out all the time you will keep warm. Unlike a drysuit, you want to have some water in your wetsuit, just not flushing through all the time.
I had trouble deciding on a size when I bought mine last winter. I usually put on 1 -2kgs over winter but I was thinner than usual when I ordered.I decided to go for a size to allow for the extra weight - although I haven't put on any since 
. I didn't get to really try it out in coldest conditions as it was late spring by the time it arrived .
Maybe it won't be quite as warm as it could be but I figured I could always wear my 1mm metallic over shirt if I was cold and at least I wouldn't have a wetsuit that I couldn't get into. 
I had to buy via the internet as I wanted a 4/5 3mm one and couldn't get any locally.
Just buy one thats a snug fit, the modern suits a stretchy and have plent of give. I have suits ranging from medium to XL, and my weight has varied to suit those sizes at some stage, however i still wear them all. Albeit some are a little tight.!
No matter how much weight you lose your bones will be the same. Same arm/leg length etc. Same distance from shoulders to crotch. So, unless you've got a mega gut and lose that, your wettie size will stay the same.
