I don't know if Dreamweaver CS5 puts a limit on the alt text for sure. I'll check the next time I use it, but I according to an article by Larisa Thomason the alt attribute has a limit of 250 characters.
It's best to keep your alt text short and sweet while conveying meaning to your visitors. Jeffrey Zeldman, founder of A List Apart, says in his book, Designing with Web Standards, Third Edition,
Use alt attribute values that convey meaning to your visitors, rather than meaning to you and your colleagues. For instance, on a logo that also works as a link back to the home page, use alt="Smith Company home page" instead of alt="smith_logo_rev3" or alt="Smith Company Logo". To a visually disabled user, it is of scant interest to be told that an image she can't see is a logo. The fact that clicking the image will take her back to the home page is far more significant.
He goes on to say a bit more about the alt tag.
Chiefly, alt text is an accessibility tool, not a nifty tooltip gimmick. (The title attribute is fine for nifty tooltip gimmicks.)...the W3C explicitly states that alt text should be visible only when images cannot be viewed.
I took a quick look at some W3C documents regarding the length of the alt attribute but it seems that they never specified a maximum length so my guess is that if there is a limit, it's imposed by the browser makers. However, the W3C wants you to keep your alt text short. They created another attribute, longdesc, for longer descriptions of images.
According to the W3C: "When a short text equivalent does not suffice to adequately convey the function or role of an image, provide additional information in a file designated by the "longdesc" attribute"
If you're trying to push the limits, go with title or longdesc instead of alt.
What is needed is a tool tip and there are lots of widgets out on the net for those. Milonic Menus has one that works great,
I have a question about alt tags and title tags. Both are needed for firefox and ie browsers since one shows the alt and the other the title. is there an extension that will change the properties area to show a fill in window for the alt AND the title? Currently if you are looking at images all they show is the alt tage cell to fill in but you then have to manually insert a title tag in the code. It would be so cool to have an option in dreamweavers settings to allow the alt tag fill in cell to also create the same information in a title tag. Saves time and keeps you from forgetting to do that.
Just an update. I tried adding alt text in CS5's Design Mode through the Properties Panel. I tried to add more than 250 characters and it works no problem. Didn't try to find an upper limit though. Of course, it's difficult to see anything more than a couple of words in that textfield. Code view is a lot better in that respect, because I could see the entire set of characters.
Having a title field in the Properties Panel might be convinient since the title tag is so often used in good coding. Until that happens, you can right-click on your images and open the Tag Editor Window, where you can find a space to enter the title information in the Style Sheet/Accessibility Panel. You'll also find a field for the Long description.
Edit: You can also find a place to enter the title information if you open the Tag Inspector Window (Window > Tag Inspector) and expand the CSS/Accessibility panel.
Edit (again): Come to think of it, the way dreamweaver handles the alt and title tags conflicts with what Zeldman states in his book. According to Zeldman the alt tag is an accessiblity attribute while the title is a substitute for the tooltip. The odd thing about Dreamweaver is that it makes it easy to enter alt text through the properties inspector but not the Accessibility Menu of the Tag Inspector or the Accessibility of the Tag Editor Window. Instead it puts the title tag in those areas. I'm okay with putting the alt tag in the Properties panel because adding alt text is considered a Good thing to do and having it there makes adding it easy, even if its not in the accessibility menu. However, I think it's a bit confusing to have the title tag in the accessibility menu and not the general area of either the Tag Inspector or the Tag Editor Window or the Properties panel.
Perhaps someone from Adobe might clear up their reasoning for this. Perhaps some research into the W3C's thoughts on the title tags accessiblity properties is needed (on my part).