that 25amp current sounds about right. 300/12=25. technically we might go with 300/10.5=28.5amps..some may even say to figure in any "surge" ratings into that as well and that might be as high as 3x, that would be 900w/10.5v=85amps! YIKES. (only a few seconds)
our handy pdi manual from sei.. (isbn#978-0-86571-520-2) gives us 4 feet with the #10 and 6 feet with the #8 for a 2% voltage drop in a 12v system.
a lot may depend on what terminal lugs are available in your area..in my area i would end up using an awg#2 wire because those terminals are readily available here with 3/8 and 5/16ths holes and the #2 can be had for less than a dollar a foot.
so i vote for #8 or better with "proper" terminal lugs (crimp AND solder) i'd stay away from those set screw type terminals, they suck for the inverter runs..and most everything else.
always interested in what other folks are working on, it's all good fun! :D
do tell us more.