10
edited after realizing you are probably talking about 2.0.11 version 
If you
are using the 2.0.11 version, in the file edituser.php replace:
Quote:
$bio_tarea = new XoopsFormTextArea(_US_EXTRAINFO, 'bio', $xoopsUser->getVar('bio', 'E'));
With:
Quote:
$bio_tarea = new XoopsFormTextArea(_US_EXTRAINFO, 'bio', $xoopsUser->getVar('bio', 'E'),10,50);
10 and 50 stand for number of rows and size of the textarea. You can change them to any value you like.
-------------------------------------
If you are talking about the XOOPS nightly version...
If you change the values for textarea in the file kernel/profilefield.php in function getEditElement(&$user), this will apply to all profile fields of type textarea.
But if that is not a problem for you, you could change the values 10 and 50 in the 'case "textarea":' section to change the number of rows and the size.
