is there a way to limit the number of characters in the preview,

In templates/default/subtemplates/posting.tpl.inc, replace...

{$preview_text}

... by ...

{$preview_text|truncate:500:"..."}

(untested; 500 is the number of characters)

as-well as the size of the image?

Hm, more complicating. Would require some modification in includes/posting.inc.php (or maybe with CSS? max-width/max-height for images within the preview box?).

Alex