What photo size can I use with my Theme optimally for the portfolios?

Hi,

For suggested image sizes please see the following: Please go to Appearance > Editor and open Functions.php file and find

$avia_config['imgSize']['widget'] 			 	= array('width'=>36,  'height'=>36);						// small preview pics eg sidebar news
$avia_config['imgSize']['square'] 		 	    = array('width'=>180, 'height'=>180);		                 // small image for blogs
$avia_config['imgSize']['featured'] 		 	= array('width'=>1500, 'height'=>430 );						// images for fullsize pages and fullsize slider
$avia_config['imgSize']['featured_large'] 		= array('width'=>1500, 'height'=>630 );						// images for fullsize pages and fullsize slider
$avia_config['imgSize']['extra_large'] 		 	= array('width'=>1500, 'height'=>1500 , 'crop' => false);	// images for fullscrren slider
$avia_config['imgSize']['portfolio'] 		 	= array('width'=>495, 'height'=>400 );						// images for portfolio entries (2,3 column)
$avia_config['imgSize']['portfolio_small'] 		= array('width'=>260, 'height'=>185 );						// images for portfolio 4 columns
$avia_config['imgSize']['gallery'] 		 		= array('width'=>845, 'height'=>684 );						// images for portfolio entries (2,3 column)
$avia_config['imgSize']['magazine'] 		 	= array('width'=>710, 'height'=>375 );						// images for magazines
$avia_config['imgSize']['masonry'] 		 		= array('width'=>705, 'height'=>705 , 'crop' => false);		// images for fullscreen masonry
$avia_config['imgSize']['entry_with_sidebar'] 	= array('width'=>845, 'height'=>321);		            	// big images for blog and page entries
$avia_config['imgSize']['entry_without_sidebar']= array('width'=>1210, 'height'=>423 );						// images for fullsize pages and fullsize slider

adjust as needed and then regenerate thumbnails using – https://wordpress.org/plugins/regenerate-thumbnails/

Best regards,
Jordan Shannon