Did you check the settings in the postfix main.cf?
In Grommunio by default the parameter "message_size_limit" is set to 0.
(message_size_limit=0)
I don't know if "0" turns off the message_size_limit or sets it to standard of 10240000 (app. 10 MB)
You can try to increase the value to your requirements and see if it helps.
By the way: I personally would not exceed a message size of maximum 25 MB. To transfer bigger files there are other suitable options available. Additionally most recipients will not accept attachments of that size you like to send.
Check also the send and receive quota limits in the user settings in the grommunio admin-ui. The settings for send and receive are only for the size of the mailbox and not for single messages! I also fell into the trap at the beginning, because I thought that this affects the size of individual messages.