Can I know my Sharepoint Site's size?

Of course you can, dear!

ONLY CLICKS AWAY - Without using Powershell and stsadm:
1) Through central admin:
Central Admin -> Application Managment -> 'Quotas and Locks' under SiteCollection > Scroll down to the 'Site Quota Information' , you will find the 'Current storage used'
2) Through the site 
Site Actions -> Site Settings -> 'Site Collection Web Analytics reports' under 'Site Actions'  

TYPE AWAY - Using Powershell or STSADM:
1) stsadm:

stsadm - o enumsites -url "http://SiteURL" > c:\sites.txt

The resulting file contains the total size.

2) Powershell
Refer 
http://blogs.technet.com/b/fromthefield/archive/2013/07/25/windows-powershell-script-to-output-site-collection-information.aspx
http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=270 ( its for Sharepoint 2010 but the solution can inspire you to tweak your solution for Sharepoint 2013)