absolutely the size that gets you.
thats why when i ship a gun i try to make the package as short as possible and (like was mentioned taking the stock off)
and well padded on the ends.
i have tried to ship a gun before and the lady said "if it was a 1/4" shorter it would ship for $30 instead of $55 or $60
so i went outside and very gently squeezed down the package a bit against the building walked back in and ....bam.... $30 shipping LOL
I used Fedex last week and it was less than I had expected. I already had one of the double boxes that a rifle had been sent to me in. Of course there are other costs to shipping, such as bubble wrap and so on. I do break down the rifles so that if a box is dropped there is less chance of it breaking the stock.