If you measure center of arm to center of arm, that is the awnings length, the fabric will be approximately 10" shorter. any supplier of awning fabric should just need the arm to arm dimension.
I replaced my own fabric this summer, ordered 17' and it was 16'-3" long.
Not hard to do, just have extra hands to do the job, two extra people is best.
Spencer & Tracie RVM#62
2013 RAM 3500DRW 4x4 Laramie, Cummins, 385HP,850FTLBS Aisin Trans
2014 DRV Elite Suite 41RESB4
Spare puller:2004 RAM3500DRW Cummins (DW's truck)