I like to take a rule from the Steve jobs playbook and use simple circles... one larger than the other but no more than 2. your most immediate target (realistic reachable) and one of the "enemy" competitor company. or overall untapped market cap.
**for this to be effective you must provide as accurate projections as possible**
no bar graphs and as little or no text as possible... remember that a deck is a companion to the speaker... avoid bullet points and use the deck to entertain rather than educate... is not a class is a pitch. :)

The actual graphical representation of the market size matters less than ensuring that the graphic is visually cohesive with the rest of the deck.

Also, ensure that you're speaking to your total addressable market, not the overall market as overall market sizing lacks credibility.

Here's some further info on calculating TAM:
http://aiti.mit.edu/media/programs/india-bms-summer-2013/materials/step_4_calculate_the_tam_---trepreneurship_101.pdf

Happy to talk through this with you in a quick call.

You should have Market Size and Addressable Market.
Market Size you can display with a large number, and then include some logos of companies and their market cap.

Addressable Market you should include a number.

If you are raising money, it is suggested your Market Size should be in $Billions.

Circles and pie charts are a terrible, terrible idea.

For your information and entertainment:

White House f*ck up with bubble charts:
http://blog.mrmeyer.com/?p=9258

A lesson on using circles:
http://www.contrast.ie/blog/infographics-and-data-visualisations/

Use the graphical way that shows your startup in best way. If your market share isn't much then don't go for options like pie chart.