Hi,
You can change image size on functions.php, find this code
$avia_config['imgSize']['widget'] = array('width'=>48, 'height'=>48 ); // small preview pics eg sidebar news
$avia_config['imgSize']['related'] = array('width'=>128, 'height'=>80); // small images for related items
$avia_config['imgSize']['portfolio'] = array('width'=>208, 'height'=>125); // small preview pic for default portfolio (4 columns )
$avia_config['imgSize']['portfolio2'] = array('width'=>448, 'height'=>260); // medium preview pic for 2 column portfolio and small 3d slider
$avia_config['imgSize']['portfolio3'] = array('width'=>288, 'height'=>170); // medium preview pic for 3 column portfolio
$avia_config['imgSize']['page'] = array('width'=>608, 'height'=>200); // image for pages and one column portfolio
$avia_config['imgSize']['featured'] = array('width'=>928, 'height'=>350); // big images for fullsize pages and fullsize 2D & 3D slider
$avia_config['imgSize']['portrait'] = array('width'=>340, 'height'=>390); // medium preview pic for portrait pictures
You need to regenerate thumbnails once you edit the thumbnail sizes.
Regards,
Ismael