Well, to begin with, a single large image for a page background could be a big mistake since unless you degrade it quite seriously it will be a large file and will slow the page load speeds right down.
There is no real rule for background tile images on web pages. It is often more important to get the pattern, if you use one, positioned correctly so that when the tile is repeated over the full size of the page, you can't see it's joints.
I sometimes use 32x32 pixels for background tiles or up to 100x100 pixels, depending on what I'm putting on the tile to begin with, but I nearly always save the background as an optimised .gif file. Then again, I rarely use a background tile to begin with, since I prefer solid background colours (usually white) rather than an image.
I suspect that your problem lies with NetObjects method of formatting its Master Border and relative page sizes, which is possibly where your backgrounds are being thrown out of perspective. Either that or you are trying to play with the background image as a page element without embedding it into a Style.
Have you tried browsing the site styles, create a copy of the style you want to use, then double click on the page background and select the background image you want to use using the built in browser ?
This is how you can build up your own styles by either modifying the existing styles or creating all of the graphics for a new style yourself then building it up from a blank.
I've just changed one of the NetObjects site styles to use a 100x100 pixel background image of my own making and it has repeated it perfectly over the page background without changing the rest of the page.
Try editing a copy of the style you want to use and load up your own background this way.