I am new to joomla and maintening a developed website.
How would I know the size of entire website that include database and all the file used in joomla.
This information is not available through the Joomla admin panel.
You can get this information very easily using cPanel.
The following answer is based on a standard cPanel setup for a single Joomla site. If you have multiple Joomla sites on the same hosting account (e.g. under addon domains), you'll have to adjust these steps accordingly for the file folder under which your specific Joomla account is setup.
Most cPanels, under the Stats section, will have your (total) Disk Space usage for that particular hosting account.
For a breakdown of sizes for the MySQL database and Joomla files used, follow these steps.
In cPanel, under 'Databases,' if you click on 'MySQL Databases,' you'll get the size of the particular database attached to the Joomla site in question (this is handy especially if you run multiple Joomla sites (hence multiple MySQL databases)) on the same hosting account.
Also in cPanel, under 'Files,' if you click on 'Disk Space Usage,' you can get a breakdown of file size by folder. Look for the 'public_html' folder and this will give you the size of the web site as per disk space used on your hosting account. (This assumes that you have not placed files that belong to your Joomla site above the 'public_html' directory), or have additional Joomla sites setup under subdirectories.
Add up the file size of the MySQL DB, and the file size of public_html, you and you will get the total size of the entire web site.
If you have a hosting reseller account, you'll likely have WHM, from which you set up new hosting accounts.
If you have a standard setup of 1 hosting account per Joomla site, then you can get the total size on disk used by that particular hosting account, including the related database.
To do this, simply go to 'Account Functions,' select 'Modify Account,' and select the account which hosts your Joomla site. This will provide you with 'Disk Usage' data for that account (including the Joomla files and MySQL DB). However, this also includes files on your hosting account not used by your Joomla site, unless of course, you've deleted those.