Add size attr to manage the size (in col) of each message.

[sebastienroul]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

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• No

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• It's submitted to the dev branch and not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested with all Quasar themes
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If adding a new feature, the PR's description includes:

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Add a new attribute to set the size of the message. Today, the size is automatic, then I let it automatic if not set. If arrt size is specified : from 1 to 12, the col-X class is added to set the size.
Very usefull for enterprise/business message displayed on desktop mode (ie. large width for the chat section) - it make the reading more confortable.

[rstoenescu]

Thanks for also doing a doc PR!