It is possible to disable the size limit in the Displace Tool, it is in the Advanced Display section. Another option is to use the Displace Tool with Displacement Intensity set to 0 to subdivide your initial mesh a few times, and then run it again w/ the actual displacement, this will allow you to go to a higher level. (We have the limits there because it is very easy to accidentally type in a number that will consume all the RAM on your computer)
