On the proxy management GUI, under Statistics > Systems > Resources > Disk Use, there is statistics for 'Disk available for cache' but there is no statistics for the actual disk size been used by the cache.
To view the actual cache disk size been used, go to this advanced URL - "https://ProxyIP:8082/CE/Statistics" and look for 'Disk in use by cache'.
The value is in 'blocks', where a 'block' is defined as 8192 bytes.
In the example below, the 'Disk in use by cache' size is 9305305 x 8192 = 76,229,058,560 bytes, which is equivalent to about 76.2 GB.