Calculating market size for a CRM app?

How can I figure out my market size for a CRM app similar to ZenDesk (support desk)? I am creating my pitch deck and can't figure out to accurately calculate that.

The snark in me would suggest that if you don't know this, or where to find it, or how to estimate it, you don't need to be building a pitch deck.

In the interests of being useful, I typed "CRM market size" into Google and found this article, which has the answer you're looking for:

http://www.forbes.com/sites/louiscolumbus/2013/04/26/2013-crm-market-share-update-40-of-crm-systems-sold-are-saas-based/

I tend to agree with Nicks answer - with the additional comment that a deck should be more specific and referencing TAM SAM and SOM. Don't know what that is? Read here.