I am planning to purchase a strain gauge, the BF350-3AA in particular although there is no mention of how much force it is able to withstand. Also if someone could advise me of other strain gauges. And are strain gauges used weight scales (the ones you stand on) or is there some other component to measure force.
Straing gauge do not measure weight, they measure strain (deformation). According to manufacturer's datasheet (http://www.elecrow.com/download/Coding%20System%20of%20Strain%20Gauges-AGS-TECH%20Version.pdf), the BF series can handle strain of up to 2%.
In the scales, the strain gauges are attached to a beam which deforms under weight. The whole assembly is called 'load cell' (sometimes also 'force sensors' and 'load sensors'), and these are rated for specific weight.
Example: http://www.robotshop.com/en/sfe-load-sensor-50kg.html. This one can accurately measure up to 50kg, and can handle up to 150% of that amount (75kg). The website claims it came from bathroom scale.
As everyone is pointing out, strain gages measure deformation. That said, the instant you bond a strain gage to a substrate, you are measuring the deformation of that substrate in response to an applied force. The ratio between force/area and strain is known as Young's modulus. You can look up the Young's modulus for a material and using the physics, you can work out the whole problem -- for example, see http://www.clag.org.uk/beam.html