You can use this tool for free and this tool is easily available. If your website is loading slow you can use this tool to check the size of your website which might be large and it can lead to loading the website very slow. It can result in a high bounce rate.

The size of an average small website is 12 KB which will load very quickly. If there is very much meda such as: pics and videos on your website then it will load very slow as the size of the website is very large. Embedded media will increse the size of a website.

This tool shows the size of a website both in Bytes as well as Kilo Bytes(KB).

If the space is limited as per your web hosting plan then find a better web hosting plan which provides best hosting at lower price. Also keep tracking that how much space is being used in assessing each web page. The important thing is that limit the size of each page of your website so that it load faster and it also lowers the bounce rate of the website.