Are you sure it is cut off on the product detail page (controlled by product.tpl)? Or do you mean on the products list page (controlled by products.tpl)
Look in skin1/customer/main/products.tpl and find the line that looks like this:
Code:
{$products[product].descr|truncate:300:"...":true}
Change the numeric value of
truncate:300 to something larger to suit your needs, for example
truncate:1000