I feel very confident about doing this anniversary cake. However I need help the lady forgot to order her cake for her parents anniversary party for 125 people. I now get lucky to do this. However she just wants a simple cake. What size sheet cake would work for this. I normally do tiered cakes. Also what would be the price for this? 

If I understand what you are asking, sheet cakes are generally cut in 2x2 pieces so the following is a chart:

full sheet: 96 pieces - but pan won't fit in standard home oven

1/2 sheet:  48 pieces

1/4 sheet:  24 pieces

As far as price...it depends on your area, the detail etc.  In my area a simple 1/2 sheet cake with a rosette on each piece runs around $85-$100.

Thank you for your response. I am not using my home oven to make it I actually work at a bakery and they will allow me to use our oven there. 

Do you have any suggestions what you would use to make a cake to feed 125 people? I think it would be easier to do a simple 3 tier cake.

Yes, I agree a simple 3 tier would be better.    I suggest  14; 10 & 6  rounds  (if I remember the # of servings right, a 14 should serve around 74; the 10 serves 25 and the 12 or 14)  rounds.   

Pricing is another matter :(   As the other poster said, it depends on your area.......but not less than $4 per servings sooooo looks like your price should be around $500.