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The file you're looking for is:

/app/code/core/Mage/Page/Block/Html/Pager.php

The line you want to modify to adjust review pagination limits is this one:
protected $_availableLimit = array(10=>10,20=>20,50=>50);

Replace the numbers with the review page limits that you'd prefer.

Of course, since this change is in core code, you'll first want to copy this file into /app/code/local/Mage/Page/Block/Html, and then make your change, to ensure that upgrades do not override it.