I'd agree with others that any fabric store that won't let you measure doesn't deserve your custom. :shock:
Another tip is to take two small mirrors with you. If you hold them at various spots over the fabric like an open book, it'll give you an idea of the kaleidos you'll get.
Online, some sites tell you the length of repeat or at least give an image with a tape measure alongside so you can judge. In the past, whenever I've emailed a supplier to ask, they've always replied. Here's some links, happy shopping! :D
http://www.jandofabrics.com
http://www.virginiaquilter.com
http://www.canadianquiltshop.com/by-...c-4_6_205.html
http://www.equilter.com/