Ot: how email size is established?

Hi all,
mostly asking here  because of all the bright minds working in various 
formats.
Tiny background, because of  the many things I do professionally, I often 
need to send files, to others, to my office  email, to robobraille.org so 
they can be converted to other formats, to staff for editing and the like.
For well frankly decades this has not been an issue..until now.
I am sending a file that is slightly larger than 7 meg.
However I am getting a users.shellworld.net error claiming that  the file 
exceeds the slightly above 10 meg size restriction.
Odd thing is that I have sent the same file previously, with no error at 
all.
what I am seeking as a short term solution is a way to understand how 
programs decide email size...even if the email itself  is entirely blank, 
I am getting the error.
No idea where 3 meg or so is coming from  and cannot plan accordingly.
Also cannot reach our administrator, so again seeking to discover a 
solution.
ideas?
thanks,
Karen