Short and Feature are nothing more than descriptions of length... Short is < 30 minutes, feature is > 60 minutes (in most descriptions)... there are variations as well: Long Short or Short Feature describe things in that 30-60 minute range. One page of a properly formatted script =~ 1 minute of screen time, action will be slightly longer and dialog slightly shorter.
When choosing a script, I first look at the page and visually scan for the balance of dialog to action (just non-dialog, not necessarily explosions) blocks. A nice blend with short (no longer than 3 sentence) pieces of dialog will prompt me to actually give the first 3-5 pages a read through.
Glaring grammatical errors or typos (depending on the experience of the writer) will close the file for me and I won't read past those 3 pages. I take notes as I read through the first time with my initial reactions to anything in the script, questions I have about plot or character inconsistencies/ vagaries, or anything else that comes to mind during this first time through. If the story is solid and the script is financially and technologically approachable from a production standpoint, I then consider the script produceable.
I've just gone through this process after making a request on this forum for scripts. I received about a hundred offers from writers of various levels of ability and I've read precisely 2 full scripts using this method. If it seems harsh, the reality is that the writer needs to write in a way that doesn't waste the producers' time and the producer needs to be able to maximize theirs so that they can get on with producing. Many of the offers I couldn't get past the first 2 pages, the scripts were huge blocks of dialog with no action blocks at all on a visual scan of the whole document and the dialog was completely expository with no hint of plot give outside the dialog. Many of them were poorly written from a grammar and spelling standpoint as well making the time I did spend on them difficult at best. (If you're going to claim to be a writer, please learn the language in which you're writing - in these cases, English - there's rule books out there for you to follow and classes to take that can help you out - and a pocket dictionary should be sitting next to your computer at all times like it is on mine and I don't claim to be a writer).
As a producer, I need to budget my time wisely, having an initial check list that gets the difficult reads out of the way helps do that. Do I miss great stories? I don't know, but I know that what I miss in possibilities there I save in Tylenol from trying to make my way through some of the scripts I've received.