I found a partial way for resizing:

first in tiki-pick_avatar.php
find the line containing



if ($iwidth == 45 and $iheight == 45) { 





and replace it with

if ($iwidth == 100 and $iheight == 100) { 

second:

in lib/tikilib.php

find the following



case 'l':

		$ret = "<img border='0' width='45' height='45' src='" . $libname . "' " . $style . " alt=\"$user\"/>";
		break;



case 'u':
		$ret = "<img border='0' width='45' height='45' src='tiki-show_user_avatar.php?user=$user' " . $style . " alt=\"$user\"/>";
		break;
	}




and replace again every occurence of 45 with 100. By now you've probably caught the idea, instead of 45 there can be any value but they should be the same to keep the picture square. I uploaded an animated gif of 200x200 px and it resized perfectly.
And here lies the real problem. Tiki was designed with square avatars in mind. Meaning if you upload a 200x150 px avatar it will get resized to 100x100 px, meaning it will get really distorted. But that's as far as I can go. My PHP skills are pretty limited, so rewriting the code that resizes the avatars so that it keeps proportions for non-square avatars is beyond me. Maybe one of the developers will take on this task for tikiwiki 1.9.1 or something like that.