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They often use a large databases whose size is tens of gigabites.

Is my sentence is correct?

If so, how can I convert the whose clause to an adjective positioned before database?

They often use a .......... sized database.

Thanks.
Comments  
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Is my sentence is correct?-- Yes, if you mean from 20-90 gigabites. (Omit the indefinite article, though.)

If so, how can I convert the whose clause to an adjective positioned before database?-- As above and below:

They often use a 20- to 90-gigabite database.
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Great! 
Thanks, Mr M.
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Liveinjapan tens of gigabites.

I would say dozens of gigabytes.

CB
Thanks, CB.
Just as 'tens of' means 'dozens of', 20 to 90 gigabyte database and 10 to 100 gigabyte database are about the same in general, right? 
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It may depend on how the databases are built-- if they are built in decimal units only (20, 20, 40), 'dozens' wouldn't sound quite right; if they are composed of any number of gigabytes (35, 42, etc), then 'dozens' is a more natural choice.
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Understand!
Thanks again, Mr M.