The purpose of trademarks and service marks is to identify the source of origin of various goods and services. By observing the trademark, consumers know who produced the goods or services they are buying. Thus, one would think that the larger the trademark the better the recognition. The U.S. Patent and Trademark Office, however, does not always see it that way. Particularly with clothing, a very large use of the trademark on the specimen is likely to prevent registration.
It is common on shirts and sometimes pants and other pieces of clothing to have a large version of a trademark. For example, a shirt may have a large image of a Nike Swoosh or a Polo player. Most people readily identify those products as coming from NIKE or POLO, respectively. Rather than viewing this as acceptable use, however, the Patent and Trademark Office considers this use to be ornamentation. In other words, the image is functioning as a design on the goods rather than an indicator of source. The rationale for this position is somewhat strange, particularly with known marks. For example, who would use a very large image of a person riding a horse with a polo mallet other than the POLO brand from Ralph Lauren. Nonetheless, that is the position the US Patent and Trademark Office, and one which clothing manufacturers need to be aware. If you intend to use a large image of the trademark (i.e. anything that would fill up more than half of a shirt pocket) it would also be a good idea to use the image on the label of the clothing. That way, a photograph of the label can be submitted as a specimen, thereby avoiding an ornamentation refusal from the US Patent and Trademark Office.