Assuming you're using an S60 phone, I'd suggest you to download and install X-plore or Y-browser. The latter can be downloaded from drjukka.com for free. If you run through the settings, you can find options to enable viewing ROM drives. If you select its properties, you can find the size, number of files, folders, etc.
Cheers,
DeepestBlue
5800 XpressMusic (Rock Stable) | N73 Music Edition (Never Say Die) | 1108 (Old and faithful)
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