I find it very odd that your file sizes are too big as all slideshow software and video editing software that I know of will automatically handle differing file sizes when creating slideshows.

what exactly are you planning to do in the slideshow. I make slideshows all the time for work and mostly use adobe premier pro. It has an automatic feature that will scale a photo or selected photos to the frame size you are using. this can save you hours of work resizing copies of your originals.

You can of course resize your pictures to the default frame size you are using. (this is going to vary if you are exporting as a widescreen or 4:3 video and what format you are exporting (pal, secam, ntsc, hd or blue ray) photoshop has some automated tools that will resize a bunch of photos automatically, however if you don't have photoshop almost any image editor will do it for you. when using this method i like to make my photos ever so slightly larger then my output size so that I have a bit of room to zoom in our out and animate photos by moving them left or right so that the slideshow doesn't feel so static.

I suspect however that you are not using the right software for the job. If you run a pc as long as you have a licensed copy of windows you can download photo story from microsoft for free. if you are on a mac iphoto will automate the process of making a slide show for you anyway. I still prefer to use a video editor such as premier pro, vegas, final cut etc but these are expensive and can be complex to use for the uninitiated. both photostory and I photo take a lot of the hardwork out of making slideshows and still offer a suprising degree of control. The one downside to photo story is that it saves the video as a .wmv file wich most dvd burning programs don't like. The simplest way around this is to open the .wmv file in movie maker (also free if you have windows) and save it as a .avi file in the format that you will use for your DVD. In my case this is DV AVI PAL, but will probably be DV AVI NTSC if you are in North America. You will then need to load this file into your dvd burning application before you can watch it on your dvd player. if you are happy to watch it from your computer there is no need to convert the .wmv file to .avi

let us know how you get on.