logistics is a set number my friend, as you build units you use up the logistics until you hit the cap, youll need to scrap some divisions that you dont need anymore to build any new units.
youll see the logistics cost on each unit right above the purchase button.
Also note that airplanes and tanks are high logistic cost, infantry are cheap logistics wise, so maybe you can relayout your armed forces next game a little.
as far as i know you can not scrap naval units, or you cant while at war? havent checked into the rules as of yet, but definitely wont let me scrap while at war.
My only thought on ship scrapping is A) it cant be for much percentage of the ships original cost or else britain and USA would get a substantial little boost to the 1939 scenario if they wanted.
or
B) when you disband ships they take 10? turns to break down and give you a slow influx of resources and strength.
I bet at one point they were allowed to be scrapped but people abused it and made their land game that much stronger earlier due to quick available resources.