BuddyPress.org

It works e.g. with:

#buddypress ul.item-list li img.avatar {
    width: 45px;
  	height: 45px;
}

Note:

1st #buddypress ul.item-list li img.avatar is also used for the photos within the member list. – So, I decided finally for 80px for the first pictures within in the activity list, and for 90px for the second photos there as well as for the photos within the member list.

2nd I tried to change within the member-loop.php

 			<div class="item-avatar">
				<a href="<?php bp_member_permalink(); ?>"><?php bp_member_avatar(); ?></a>
			</div>

into

 			<div class="item-avatar_2">
				<a href="<?php bp_member_permalink(); ?>"><?php bp_member_avatar(); ?></a>
			</div>

and to define within the ‘Simple Custom CSS’-Plugin:

#buddypress ul.item-list li img.avatar_2 {
    width: 110px;
  	height: 110px;
}

But this didn’t work.