Estimating the height of vegetation is extremely complex.
Classical radar interferometry is able to measure height of the terrain (as done by the SRTM or TanDEM-X missions). A very nice example using Sentinel-1 data is given here
These principles won’t work for vegetation bodies because phase information decorrelates with subtle changes of a surface within a short time. This is especially the case for vegetation (growth, movement, ect…).
Height of vegetation requires the application of more advanced techniques, such as polarimetric interferometry (PolInSAR) or radar tomography, but these are not featured in SNAP. Furthermore, you need fully-polarized data (HH+HV+VH+VV) and Sentinel-1 is dual-polarized only (VV+VH).
Here is an example how vegetation height is retrieved:
- Estimation of Rice Crop Height from X- and C-Band PolSAR by Metamodel based Optimization
- Related topic in this forum: Canopy/vegetation height