Pricing for 6ft by 8ft painting?

I am horrible when it comes to pricing, I have no idea where to even start.
I have a client I’m working for, he wants me to make a 6ft 8ft painting for his studio. It is the largest painting I’ve ever done. Two large koi fish with no background at all. A fair amount of detail, scales with some shading and highlighting. I’m scared to charge him too much and need to talk to him soon. Is $900 to $1,300 too much to ask for??