I myself bought a F800GSA 2 months ago and it is my first motorcycle. Prior to the GSA I have never ridden motorcycles besides the mandatory lessons to get my license.
I have currently ridden about 2.000km mainly commuting to work and back. I did manage to drop it once at an intersection without any real damage, which I'm sure could have been prevented if I had started with a smaller/lighter bike. I currently ride 99% on road, where the weight/size of the bike is less of an issue than offroad.
As long as you are careful and realise it is a fat bike and you will drop it (several times), I think there's no real issue with starting with a F800GSA. A lot of it comes down to common sense and a bit of luck. Just don't get yourself into situations you can't handle yet.