Before you run the commands you should check the Transport settings config and check the individual mailbox settings also.
This command is to check the transport config
get-transportconfig | ft maxsendsize, maxreceivesize
This command is to check the send and receive max for individual or all users
get-mailbox “Administrator” |ft Name, Maxsendsize, maxreceivesize
Put an individual user in the quotation or to see all users remove the user and quotations.
This command will set the transport
Set-TransportConfig -MaxSendSize 15MB -MaxReceiveSize 15MB
This command will set all the individual users
Get-Mailbox “Administrator” | Set-Mailbox -MaxSendSize 10MB -MaxReceiveSize 10MB
You can use it on an individual user or remove the user and quotation so it runs on all users.
Change the number accordingly.