sum of column in entity framework

hello,

I have following table in sql and I want to sum FileSize column for particular UserId in asp.net web api core entity framework using Lambda expression or Linq. Filetype of FileSize column is Double and  Table is :

PostId    UserId  FileSize

1             2             1

2             2             3

3             1             1

4              2            2

So I want to calculate Filesize of User Id 2. i.e Total is 6

Thank you