Click somewhere in the chat window (not on someone's name), and the window options should appear to the top left. Right click the tab, click Unlock Window, and you can resize the window my left-click-dragging the lower right corner of the window, or reposition the whole window by left-click-dragging the tab. When you get it to the size you want, just Lock Window again, and you're good.
The two main addons I use for DK tanking (specifically, in addition to my normal addons) is IceHud and Power Auras (especially useful for Frost spec). IceHud has the option to display your runes in the middle of the screen, in addition to health/mana/runic bars for yourself, your target, even a focus target. I use it solely to have my runes right above my hotkeys (bartender4), so I have less distance to look to see which runes are up.