You can use the command split for this task. For example this command entered into the command prompt
split YourLogFile.txt -b 500m
creates several files with a size of 500 MByte each. This will take several minutes for a file of your size. You can rename the output files (by default called "xaa", "xab",... and so on) to *.txt to open it in the editor of your choice.
Make sure to check the help file for the command. You can also split the log file by number of lines or change the name of your output files.
(tested on Windows 7 64 bit)