Hi Alex,
There is absolutely no way you need 72dpi for something that big. You’d easily get away with 5dpi. Have a look at a billboard up close one day.
As a rule of thumb, the minimum viewing distance, which allows the whole image to be viewed in its entirety, can be calculated by multiplying the length of the diagonal of the item by 1.5.
Then use this handy table to work out the required dpi for continuous tone (ie. photographic) images
If you have vector art or text that needs sharper edges, then you are going to have to map that to your balloon then flatten that separately. Then colour that in Illustrator and assemble the whole horror show in InDesign.
Regarding texture size limit. That is mainly a hardware limit depending on the amount of VRAM in your video card.
If you go Tools>Options>View>OpenGL it will show you the max texture size in pixels you can use. Note that Rhino doesn’t report Vid memory over 4GB for some strange reason.
If you can’t fit your graphic into those limits, then look at multiple UV sets and deal with it that way.
Obviously there is a lot to this, but hopefully that gets you pointed in the right direction.
Steve