Richard, which is the attached file extension?
Do you use the free or the paid version of Zone Alarm?
avast does not stop sending emails by size... it could be a small timeout...
Try to increase both timeouts (of avast into the Advanced tab of Internet Mail provider settings and of your email account, in Properties of the account, Advanced tab too). You can set both to 600, for instance. If you're on a DSL connection (not dial-up) it shouldn't be necessary a so long timeout anyway...
If this does not help, can you uncheck the Outbound mail scanning?