When downloading a compliance download from Office 365, there’s a setting in the registry to control the size of the resulting PST’s.

The .reg file should look like that:

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Client\eDiscovery\ExportTool]
“PstSizeLimitInBytes”=”8589934592”

And here are possible values:

1GB 1073741824
5GB 5368709120
10GB 10737418240
15GB 16106127360
20GB 21474836480
30GB 32212254720
40GB 42949672960
50GB 53687091200

You can compute these amounts manually by doing  N x1024 x 1024 .