How to create big size billboards?

I need to creat a billboard that is 7m width and 3m height.

I started in Photoshop, and after I saw that the clean file is about 18gb, i said like hmmmm... Then i tried Adobe illustrator but it allows maximum file width 577,95cm which is 5.7m.

How can create the billboard?

Solution

Billboards that big are usually printed in very low resolution. For image that size you will probably need no more than 15-30 DPI and it will drastically reduce your file size.