I want to design some machine parts in SolidWorks. Where can I find the dimensions?
Sometimes I want to design something existing already. It may be a assembly or single part file.But I can't find the proper dimensions. Is there any website to find dimensions?
For the most part, no.
It somewhat depends on what the item is, but producers and manufacturers generally are not fond of giving away their design and dimensions to people.
Your best option is taking measurements by hand and remaking the object. If it is something very complex, then maybe you'll look at laser scanning, or photogrametry to capture the data. There are pros and cons to every method, so don't assume there is one method that works best every time.