You can use a hole burning paper to know the beam diameter.

Or use an aperture to measure at what diameter of the aperture the intensity of the laser deceases to 86.5% of the maximum value. This is based on Gaussian beam where beam size is defined as the distance where the intensity decrease to 1/e2 of the maximum. So if your beam diameter is 10 mm start with an aperture of 15 mm which will give a maximum laser power. Decrease the aperture and see at what diameter of the aperture the intensity decreases to 86.5%. This is the beam diameter.

https://hds.leica-geosystems.com/hds/en/Prof_Surv_Spot_Size_Oct06.pdf

https://www.rp-photonics.com/gaussian_beams.html

https://www.newport.com/n/gaussian-beam-optics

https://www.ophiropt.com/blog/laser-measurement/calculate-laser-beam-size/

Many thanks Tuhin Khan . Actually, I want to know the relationship between intensity of the beam versus the beam radius.

A = 3.14*(beam radius)^2