I recently got Terraria, and I'm playing it on my Ubuntu machine with PlayOnLinux. I just set it up with the nice little installer from PlayOnLinux. However, when I run it, the Wine desktop comes up in 1024x768, as well as Terraria. I've tried changing the resolution in terraria, but it stays the same size and the cursor movement is off. I have to Alt-F4 out because the cursor leaves the window before it gets to the settings button. I have also tried setting Terraria to fullscreen, but it's only fullscreen in the Wine desktop. I'm using a GTX 750 Ti.
The first step is to get the Wine desktop the size you want. Once you have that setup, you'll probably have to go change the video settings in Terraria. If the settings look correct, but Terraria is still using the smaller window, toggle the windowed/fullscreen setting once or twice and then re-select the resolution.
The Terraria developers are supposed to release a native Linux client pretty soon now, so hopefully we won't need any more workarounds like this.