The Upgrade table description on MSDN states: "VersionMax must be a valid product version as described for the ProductVersion property.". The ProductVersion property description on MSDN (http://msdn.microsoft.com/en-us/library/aa370859(VS.85).aspx) states: "The first field is the major version and has a maximum value of 255. The second field is the minor version and has a maximum value of 255. The third field is called the build version or the update version and has a maximum value of 65,535.".
This should mean that the maximum VersionMax value allowed is "255.255.65535". However, if you add that value and run validation you get the following ICE61 validation error: "ERROR Upgrade.VersionMax 255.255.65535 format is wrong".
The largest value I have been able to add to the VersionMax field and still pass ICE61 validation is 127.254.32767. This is probably an issue with the ICE61 validator itself but I thought I'd post it here in case anyone else is having this issue.