While we chatting in Google Talk it is not possible to customize text like increasing font size, type in bold or italic and write multiple lines of text in a single hit.
In this post you will know some tips and tricks for the google talk and also some key board shortcut keys usefull for the google chat users 

 * Change the font size: While holding the control key, move the scroll wheel on your mouse either up or down. This trick works while being focused in either the read or write area.

* Insert line breaks: If you want to have a message that spans multiple paragraphs, just hold shift and hit enter. You can add as many new lines as you want to create.
* Bold Text: To write something bold, you can use an asterisk before and after the word,for eample like *this is blold text*
* Italic Text: To use italics, use an underscore before an after the word,for eample like _this_
* Switch windows: Hitting tab will cycle through open chat windows. If you just want to cycle through any two windows hit control-tab
* Invitation Tips: You don’t need to say Yes or No when someone wants to add you as a friend; you can simply ignore it, the request will go away. (On the other hand, someone with whom you chat often will automatically turn to be your friend, unless you disable this in the options)
Keyboard Shortcuts:
* Ctrl + F4 – It closes the current window.
* Alt + F4 – It closes the current window.
* Alt + Esc – It Minimize all the windows.
* Windows + Esc – Open Google Talk (if it’s minimized, or in the tray)
* F9 – Open Gmail to send an email to the current contact.
* F11 – It initiates a telephonic call with your friend.
* F12 – It cancels a telephonic call.
* Esc – It closes the current window.