Depends on what format your using, but there should be selections to set the size of photo, see what the smallest is and also set your camera to take small pictures.
It depends on what program you are using to edit, but i suggest getting your business cards as the ones that are already cut -out (Avery) or having them made by professional printers.
I am sharing a link which will helps you to reduce business card. This company is good in printing.
Solve this dilemma by choosing the the right online printing company. Printrunner offers free file review wherein their prepress team can do image adjustments for your business card design. http://www.printrunner.com
Photoshop is a software which is just perfect to re-size such kind of pictures. If you are not familiar with Adobe Photoshop then I am suggesting you a Youtube video which can help you to re-size an image so that you can fix that on a business card : http://www.ehow.com/video_4467307_resize-images-photoshop.html
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You'll have to adjust the picture using an image-editing software such as Photoshop. The quality of the image might suffer if it's smaller than the size you wanted. You may also have to do a little bit of trimming to make the picture fit properly without warping it.
Business cards' main role is to let your possible target clients know what you are offering, what business you are into. It pre-sell and generate a possible communication between you and your clients should they get interested in acquiring your services/products etc. So these little piece of cards are a big help to your business basically.
Resize and edit the layout in Photoshop or any publisher program you have. Customized plastic business cards are better with catchy images, but it should highlight the important elements like contact details etc.
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them