The basic OT release (tarball) takes up about 100 MB of disk space, but is significantly larger (200 MB) if Java is included. If the new installer is downloaded the required space is closer to 300 MB.
The default memory allocation for the OT is 4 GB, but this can be changed using the OT Installer. This large amount of memory means that only 64-bit versions of Java can be used with the OT.