All image sizes are defined in functions.php:
$avia_config['imgSize']['widget'] = array('width'=>36, 'height'=>36); // small preview pics eg sidebar news
$avia_config['imgSize']['slider_thumb'] = array('width'=>70, 'height'=>50 , 'copy'=>'greyscale'); // slideshow preview pics
$avia_config['imgSize']['fullsize'] = array('width'=>990, 'height'=>990, 'crop'=>false); // big images for lightbox and portfolio single entries
$avia_config['imgSize']['featured'] = array('width'=>1140, 'height'=>461 ); // images for fullsize pages and fullsize slider
$avia_config['imgSize']['portfolio'] = array('width'=>495, 'height'=>400 , 'copy'=>'greyscale'); // images for portfolio entries (2,3 column)
$avia_config['imgSize']['portfolio_small'] = array('width'=>284, 'height'=>211 , 'copy'=>'greyscale'); // images for portfolio 4 columns
$avia_config['imgSize']['logo'] = array('width'=>446, 'height'=>218 , 'copy'=>'greyscale'); // images for dynamic logo/partner element
//dynamic columns
$avia_config['imgSize']['dynamic_1'] = array('width'=>446, 'height'=>150); // images for 2/4 (aka 1/2) dynamic portfolio columns when using 3 columns
$avia_config['imgSize']['dynamic_2'] = array('width'=>609, 'height'=>208); // images for 2/3 dynamic portfolio columns
$avia_config['imgSize']['dynamic_3'] = array('width'=>688, 'height'=>150); // images for 3/4 dynamic portfolio columns
Depending on the number of columns different thumbnail sizes are used. You can change the height value(s) btw - so if you're unhappy with the current aspect ratio you can adjust the height value(s) to your needs. You can also deactivate the cropping feature by appending 'crop'=>false to the code. A plugin like: http://wordpress.org/extend/plugins/simple-image-sizes/ adds a nice UI to the wordpress settings page and you don't need to modify/edit functions.php. If you change the image sizes you need to regenerate the thumbnails with: http://wordpress.org/extend/plugins/regenerate-thumbnails/