Hi guys I want to install a double din head unit but unsure what size the screen should be anyone help me...

Double dins are a standard size mostly, some have smaller screens, they probably have buttons down the side etc, and a bit cheaper, 7" or even 8" now, are touch screens, there is no room for buttons (almost), so touch screens are in.
So you are pretty free to pick choose, just check back in with da Boyz, when you have made a selection, if it helps put you at ease.....

DIN is an international ISO standard and originally a German standard (where the DIN name comes from.) It sets a specific width and height for head units. It doesn't set a depth, but I've never found one that won't fit. If it's advertised as double DIN, it will fit. Screen size shouldn't make a difference, although you'd obviously be limited by the size of the standard.

Hang on to the mounting plates from your original unit if it's a double din, as they should be able to screw into your new one, it makes mounting a lot easier.