File/attachment size limit isn't a configurable option in HipChat Server admin interface at the moment however you are welcome to vote up the public feature request about this matter here: HCPUB-374. There's a user who shared a workaround for that however it may work only on web client since there's a hardcoded limitation enforced on the desktop client.
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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