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)

Show me the full error!!!

"Yeah! Right... Show me the error! I want it now! Fed up of you sending me chasing after a stupid Co-relation ID!". Do you relate to this desperate call of frustration? Then you have come to the right place.

The error message isnt very helpful. Plus, this error message is ok if you had easy access to the server or atleast someone sends you the ULS log the moment the error occurs. But in the cases that both dont happen, its best you have the full error message displayed, at least till the issue is resolved. 

To enable the whole error message in Sharepoint 2013 just make these two changes to the web.config file:
1) Enable Custom Errors: 

Change to:

<system.web> ...

<customErrors mode="Off" />

2) Enable Call Stack Trace: 

Change To:

<SharePoint>

<SafeMode ... CallStack="true" ... >

</SharePoint>

Check if the error message is now showing as expected.

Doesn't it make you happier? Happy trouble-shooting!