dose anyone know what size nut gose on the tensioner bolt on an '04 celica?

I honestly don't know and do not have a source to check, "BUTT" and it's a big butt at that. I have purchased some good name brand tensioners that came with a new bolt in the box. If it worked loose enough to toss the belt you most likely should replace the fastener. If you call a parts store in your neighborhood they can answer if the hardware is in the box for you. Tom

The hardware on the tension are 10mm x 1.25 thread pitch.