Hi,
You should be able to change thumbnail sizes by editing the functions.php in your theme files. These are the lines, you want to look at:
$avia_config['imgSize']['widget'] = array('width'=>36, 'height'=>36 ); // small preview pics eg sidebar news
$avia_config['imgSize']['post-format-image'] = array('width'=>630, 'height'=>999, 'crop'=>false);
// big images for post format image and gallery posts
$avia_config['imgSize']['fullsize'] = array('width'=>930, 'height'=>930, 'crop'=>false); // big images for lightbox and portfolio single entries
$avia_config['imgSize']['featured'] = array('width'=>930, 'height'=>340); // images for fullsize pages and fullsize slider
$avia_config['imgSize']['portfolio'] = array('width'=>465, 'height'=>346); // images for portfolio entries (2,3,4 column)
$avia_config['imgSize']['logo'] = array('width'=>465, 'height'=>220); // images for dynamic logo/partner element
//dynamic columns
$avia_config['imgSize']['dynamic_1'] = array('width'=>446, 'height'=>151);
// images for 2/4 (aka 1/2) dynamic portfolio columns when using 3 columns
$avia_config['imgSize']['dynamic_2'] = array('width'=>609, 'height'=>210);
// images for 2/3 dynamic portfolio columns
$avia_config['imgSize']['dynamic_3'] = array('width'=>688, 'height'=>151);
// images for 3/4 dynamic portfolio columns
Depending on the size you may also need to adjust the CSS.
Regards,
Mya