There's a line across the top of the 'text' icons (Where you change font size and colour... and add Emoticons) just under the 'chat window' (This is of course provided you're not using an antiquated version of Trillian... there's no point in using old versions of free software) If you grab that you can resize the text box where you type your outgoing message...

You can also make your text line longer (So shrinking the "Send Button") by dragging the space between the two.... You can actually size the "Send" button out of existance.... if you use "Keyboard shortcuts to send IMs, you don't need the "Send button" at all anyway....