A light dependent resistor (LDR) is giving laser intenseties as analog value. The dimension of photoactive surface of the LDR got reduced with dark tape to a minimum (maybe to <500µm²). 
Two steppermotors are rotating in X and Y axis in very small steps. With that a 2D scan take place to gather data.

The Laser spot size is defined by 1/e² .
i) Subtract the noise from your 2D-dataset.
ii) Search the max. value. 
iii) "max. value" / e² gives the value of the spot-size limits.
iv) search those values in +/- X and +/- Y-direction.
v) those min./max value will give you the dimensions in X and Y.

Since this data is not converted to linear dimensions, one could use trigenometry with a distance of 4 cm from laser to photoresistor.

The ultra Device (https://hackaday.io/project/171183-the-ultra-device-runs-myos) can star and display a scan.