Find the Site collection Size and all details

Open SharePoint 2010 Management Shell / Windows Command Prompt as administrator

Enter below command to access "stsadm.exe" and press enter

cd "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN"

Type below command and press enter

STSADM.exe -o enumsites -url "<Url of the web application>"

  Command will give output where you can see site collection details like:

a.   Url

b.   Ower

c.   Content Database

d.   Storage Used MB

e.   Storage Warning MB

5.   Modify the command with text file path to get output in text file

Example:

STSADM.exe -o enumsites -url "<Url of the web application>" > c:\SiteDetails.txt