Short answer: no.
You can set message size limits in four places: globally, on Internet connectors, on virtual SMTP servers and per-recipient.
Per-recipient settings apply to all messages sent or received from that recipient, so they're not going to help you here.
Global limits apply to every message that flows through Exchange, so they will be applied to internal and external ones.
Limits on Internet connectors apply only to messages going out of your organization.
Limits on virtual SMTP servers apply to every message that goes through that virtual server.
The only way to limit message sizes for internal messages is to set global limits or per-recipient limits, because those messages don't go through Internet connectors and/or virtual SMTP servers (unless moving from one Exchange server to another). But if you set global or per-recipient limits, they will apply to external messages, too.
You can get the opposite of what you want: allow larger internal messages and smaller external ones (and this is the usual setup); but you can't have larger limits on external messages and smaller limits on internal ones.