The value is explicitly called out in the Virtual Appliance Installation Guide. Since you are needing to manage 130 APs and 2 controllers you'll need to operate the MCR at the 500 device level.

If you are talking about the primary drive and installing from ISO, you can leave at the default of 16 GB or set to 4 GB which is what the OVA would install. The secondary (data) drive is the important one to have sufficient space for long term operation.
Note, changing the drive size post-installation is not useful, AOS will not automatically resize the partitions. If you need to expand the size of the data drive, follow the procedure that is documented in the Virtual Appliance Installation Guide.
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Carson Hulcher, ACEX#110
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