There are a lot of things to consider for grass runways. The length of the grass is only a part of the concern. You also need to consider how thick it is, how wet it is, what the soil is like underneath (hard, spongy, wet, muddy, etc.) and how is the drainage. Tire size and pressure are also a big factor too.

Axle length or shorter is good. If the soil is very hard and it is thin, tall grass it shouldn't be a problem other than cleaning grass out of you landing gear and green stuff on your prop.

If you lock your brakes in clumped grass I have heard that it can build up in front of the tire and become a chock. This can cause your plane to turn or even flip over a tailwheel plane.