Mine is a 2010 Scamp
and came with the 8' awning. Sure, it sticks past the "straight" area a bit, but it sure beats losing another foot as they're pretty narrow as it is. I'd get the largest you're comfy with....and for the 13'...that will probably be the 8'er.
One thing you can do is drop a corner to help shade when the sun is coming up or going down. Either way you look at it, the small awnings are not that effective- and I've also noticed this on the larger RV's where their awnings are higher up. I personally like the "Screen rooms" available now where you have the "sides" you can add or take away. They make a nice addition for help blocking the sun.
As far as the side-mounted awnings, I really like the Fiamma (Sp?) awnings that are out now over the Dometic Catalina. They're just easier to deploy and retract with the "crank" mechanism. The Dometic acts as a HUGE window shade retracting. What if..... Anyway, I'm sure the Fiammas are heavier and more expensive. I've seen them in action though at rallies. Nice.