Raman spectroscopy is an ideal tool for characterization of strained graphene. Biaxial strain, in particular, allows for more reliable calculation of the Gruneisen parameters than uniaxial strain. However, the application of biaxial strain is rather difficult to achieve experimentally, so all previous studies reported on graphene subjected to relatively small biaxial strains (0.1-1%), in contrast to uniaxial strain above 10%. Here we report a simple fabrication technique to produce pressurized and stable graphene membranes that can support differential pressures up to 14 bar, corresponding to a reversible strain up to ~2%. We find that the Grüneisen parameters remain constant even for the largest strains achieved, in agreement with theoretical predictions. However, for strains above 1%, a distinctive broadening of both the G and 2D peaks was observed for biaxial strain. We attribute this to nanoscale variations of strain in the membrane within an area comparable with the laser spot size.