There was a similar question a couple years ago.

The accept answer has four recommendations:

• Magnification in GNOME Shell
• Zoom Options Dialog (Universal Access)
• Gnome Magnifier Review
• Cursor as full screen crosshairs

Best Choice

All four should be investigated but the third suggestion leads to a promising app: Zoom Options Dialog (Universal Access):

Figure 2. Zoom Options Dialog showing the “Crosshairs” Tab.

Crosshairs Switch

The switch labelled Crosshairs toggles the display of crosshairs.

Overlaps mouse cursor

When the Overlaps mouse cursor checkbox is checked, the crosshairs intersect the mouse pointer. When unchecked, the crosshairs end before intersecting the mouse cursor.

Thickness

The Thickness slider determines the width of the crosshairs. The range is from 1 to 100 pixels.

Length

The Length slider determines the length of the crosshairs. The range is from very short, through fractions of the size of the screen, to a length that extends to the edges of the screen.

Color

The color button launches a color chooser dialog to set the color and transparency of the crosshairs.

Step by step instructions

Step by step instructions (using Gnome 2.2) were written by the BBC (British Broadcasting Corporation).

Here are a couple of screenshots accompanying the instructions from that site:

Ubuntu Wiki has instructions for installing Orca for Unity and Gnome desktop environments here. However Unity DOES NOT have the Assistive Technology features for Magnifier and Cross Hairs. You MUST use Gnome DE for these features. Source.

Zoom Options Dialog is part of the ÆGIS (Ontario) Project. It is funded and supported by the Ontario Ministry of Economic Development and Innovation and the ÆGIS (Europe) Project. You can also check these websites for more instructions.