I think you may be approaching this the wrong way.
Why use Snagit to do screen captures when Captivate itself is perfectly capable of doing this AND it will create screen captures that can look seamless between slides?
Secondly working out what size to set your project to doesn't depend on the physical dimensions of the monitor (i.e. 19 inch or 24 inch) but rather the screen RESOLUTION in pixels and how much of that real estate is going to be available to your e-learning content AFTER you take into account a host of other factors.
One thing you SHOULD NOT be trying to do is fill up the entire screen, if a smaller size would be quite adequate. Making the content larger for no reason is just going to make it heavier and cause it to take longer to download.
To add to Rod's comment, Captivate has no 'templates' at all. If you use the software simulation work flow that comes with Captivate, the blank daughter master slide in the active theme will be used. It is very important that this master slide remains "blank".
Just a term, but 'dpi' is a printer word, you probably mean ppi: for Captivate the overall resolution in pixels is important!
I am creating an e-learning for a healthcare system. I don't have security access to record the slides in Captivate, and have to work with SnagIt screen captures supplied by someone who does have access. I'm just trying to figure out the best way to proceed given the restraints I'm working with, so I really appreciate advice from experts like yourself and Lilybiri!
OK. Well you have my sympathy. I would strongly suggest that instead of using SnagIt, you try to get the person inside the company to have Captivate installed and get them to do the captures with that. You can tell them that it will save them enormous amounts of time because all they have to do is set up a single file for each task and then just click around the screen to replicate the task.
If they can do all of this within a couple of weeks, they could just have the trial version of Captivate installed temporarily to pull it off, then send you the CPTX file for each task. You can then paste the slides from this trial version CPTX into your own.
But if you go this route, you need to make sure that you and the other person agree BEFOREHAND exactly what the capture size will be each time. And whatever you do, DON'T just make it full screen. Go for something like 1024x768 or 1280 x 720 if the screens won't fit at the smaller size.
If they refuse to go with Captivate and insist on using SnagIT, then you need to insist that they make all the captures the same size. You can set a capture area in Snagit that makes all captures the same height and width. Give them the dimensions you have chosen for your Captivate project files and tell them these are the dimensions at which all their captures must be taken. And unless they like the idea of the finished product looking like it was jerking all over the screen, tell them they must take each capture from exactly the same pixel distance from the top left corner of the software screen. (Captivate makes doing this very easy, but SnagIt doesn't.)