Thanks to J. H. Wright for posting this:

----
======
    package require tcom
    set excel [::tcom::ref createobject Excel.Application]
    $excel Visible 1
    set workbooks [$excel Workbooks]
    set workbook [$workbooks Add]
    set worksheets [$workbook Worksheets]
    set worksheet [$worksheets Item [expr 1]]
    set cells [$worksheet Cells]
    for {set row 1} {$row<=10} {incr row} {
        foreach column {A B C D E F G H I J K L} {
            $cells Item $row $column $column$row
        }
    }
    puts [[[$worksheet UsedRange] Columns] Count]
    puts [[[$worksheet UsedRange] Rows] Count]
======

----
[[Show a similar result expressed in terms of CurrentRegion 
[http://en.allexperts.com/q/Excel-1059/2008/5/VBA-script-unknown-number.htm].]]

See also the Wiki pages on "[Excel]" and "[tcom]".

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