You can try editing /inc/classes/BxDolBlogs.php
(probably a good idea to backup and download the file before making any changes first though)
Search the file for: 110
It's a fairly large file. You will find near the top:
//max sizes of pictures for resizing during upload
var $iIconSize = 45;
var $iThumbSize = 110;
var $iBigThumbSize = 340;
var $iImgSize = 800;
(you could change the 110 to larger)
And further down in the file:
$sPostPictureTag = '<div class="marg_both_left"><img alt="" style="width: 110px; height: 110px; background-image: url('.$site['blogImage'].'big_'.$sPostImage.');" src="'.$sSpacerName.'"/></div>';
(you can mess with the 110px here too).
Don't expect what you have already posted/uploaded to change to your new larger size, as it has already been uploaded and sized to 110x110. You will either have to make a new blog post, and/or edit a current post and re-upload/re-add the image, so it will be resized with your new size settings.
If it don't work for what you are trying to do simply restore the original /inc/classes/BxDolBlogs.php