From what you've mentioned a sub-site per customer sounds suitable, but you are correct in considering scalability with the figures you've quoted.
You need to refer to Microsoft's capacity planning documentation on Technet:
http://technet.microsoft.com/en-us/library/cc261700.aspx[
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The next link in particular should give you some definite answers regarding capacity and scaling:
http://technet.microsoft.com/en-us/library/cc262787.aspx[
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There's a whole range of things you can do to maximise the capacity and performance of your SharePoint installation.
One thing you can definitely look at is provisioning the Site Collections in such a way that each Site Collection has a dedicated content database in SQL Server, and maybe spanning your customer base over a number of separate site collections so you don't run into capacity limitations. Other options include disk optimization of the SharePoint databases and configuring SQL Server for Remote Blob Storage to store binary content in the file system and not the database:
http://technet.microsoft.com/en-us/library/ee663474.aspx[
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Hope this helps.