So how big should your logo be in an ad, or on your letterhead? The guideline is: size the logo appropriately for the medium it's in. Remember, you aren't selling your logo, you are selling a product or service. The logo helps Identify you as the provider of that product or service, so make it large enough to communicate that, and no more. Here are some general guidelines for different medium:
Business Card, Letterhead, Envelope, or Print Advertising
If you look through any magazine, and take a look at the logos, they are usually around an inch wide or smaller. Of course some companies with long names may run wider than an inch, so if you stay in the .75 to 2 inch range, you should be pretty safe. Letterhead and envelopes is where people frequently make their logos larger than necessary.
Posters or Signage
I can't make a hard and fast rule here, because each company operates in a different environment, but the general rule of thumb is to go out to the street or across the hall where a typical customer might be and decide if you can see the sign or poster and make out your logo. Put yourself in the viewer's shoes. A tough place to do this is billboards. I have seen so many billboards where the logo is so small, I couldn't even figure out who the company was. It probably looked plenty big on the billboard designer's computer, but from 300 yards away going 60 miles per hour, it was tough to read. If you are in doubt, go slightly larger.
Shopping bags or shipping boxes
Here generally bigger is better, and plastered all over the top and sides works best. If you only put your logo on the top of a box, and the boxes are stacked, nobody will know its your fabulous product inside. And for shopping bags, you want visibility. No one can make out a 1 inch logo on a swaying shopping bag blocked by kids running by in the mall.
If you have a specific project in mind where you want to add your logo, check out what the pros are doing on a similar application. It should give you a good idea how to size your logo. And remember, put yourself in your viewers/customer's shoes!
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