The Type of winebag you can use can depend upon the region you are in.
I'm in mid Europe. And here most wine in a bag is in a 3l wine bag in a box.
On the internet I found the measurements of the original box
Measurements Bag Box palet
Length 125 mm 125 mm 1,20 m
Depth 125 mm 125 mm 0,80 m
Height 235 mm 235 mm 1,19 m
Volume 3,67 dm³ 1,1424 m³
Wheigth 3900 gr 29680,00 gr 1360,60 kg
If you have another box you just have to calculate the measurements
5L = 5 dm3 = 250mm*200mm*100mm
In inches and gallons it should work the same.
Now you have the indication measurements for the volume
This space You will need in the bear, so now you can start looking for a bear.