In Ubuntu 10.10 I could enlarge the mouse pointer. In Ubuntu 11.04 I cannot find the option.
Due to a bug in compiz the mouse pointer size or pointer theme can be changed in Unity but may only work in some circumstances (e.g. Firefox).
For GNOME classic and Unity desktop the option to change the mouse pointer size or style is found in System Settings -> Appearance -> Theme -> Customize -> Pointer:
Push the slider at the bottom to the desired size.
Workarounds for the bug in Unity are now referred to on the bug page however your mileage may vary.
None of them had any positive effect in my system.
Unfortunate fail for Ubuntu, given that this is a longstanding issue, degrades the claim that Ubuntu is flexible or mainstream ready.