Joaquin,
In the USA cloth is frequently woven and sold in widths of 36, 44 or 45 inches (nominally 45 inches) and 59 to 61 inches (nominally 60 inches). the thickness of the fabric can be expressed by measuring it with a micrometer, but more commonly the fabric is described in terms of the weight of one square yard of the fabric. That would be 36 inches by 36 inches.

The fabric, however is sold by the running yard, so if you have cloth that is woven 60 inches wide and you buy 1 yard you'll have a piece that is 60 by 36 inches.

Since you are in Germany I assume you'll get fabrics which are specified in terms of gram per square meter.

According to this conversion chart from Fiberglast

http://www.fiberglast.com/UsefulCharts/weightsmeasures.htm

You take the weight in ounces per square yard and multiply it by 33.91 to get the equivalent weight in grams per square meter. So, 9 ounce cloth would be specified as 306 gm/sq. m. (or probably they would round it off to either 300 or 310.)

There is a lot of other good information on fiberglass, and various related fabrics and construction techniques on the fiberglast site. www.fiberglast.com

Hope this helps

PGJ