This message is shown when the image file takes less space than what it's declared in the ISO9660 filesystem descriptor. Are you sure the image is not broken?

I cannot tell because the provider of the ISO file (Microsoft ;-D) does not provide any mda5/sha1. 

But back to your comment -- if k3b is able to check the file size comparing the value in the header in the file itself then in such case the message should be a little longer and more informative. IOW: if k3b has more info it should be shown to the user, something like:
The actual file size does not match the size declared in the file header. It is very likely there was an error while downloading/copying the file. 

Btw. this message should be shown right away when picking the file, not just before burning.

SVN commit 1061711 by mmalek:

In "Burn Image" dialog, when image file size is invalid (i.e. less than ISO9660 Volume Space Size):
* display the filesize in red with appriopriate comment
* display also Volume Space Size to give user a feedback about the correct size
BUG: 216812

 M  +26 -4     k3bimagewritingdialog.cpp  


WebSVN link: http://websvn.kde.org/?view=rev&revision=1061711

I have added filesize checking earlier. If it's incorrect, the file size on the image information area is shown in red. The Volume Size is also displayed.
I haven't touched the message box, it will be the additional warning for the user that something might be wrong.

I reopen this report for a while just to save from opening another one:
a) please change the message for something more informative -- this is what this report is all about (I am not saying that my idea of text has to be used)

b) small issue, but valid -- please don't use hardcoded colors -- all colors should be symbolic, so not color red but color KDE::Negative (this is workaround anyway, because from A11Y POV such info shouldn't be represented using color solely, there should be added an icon or text)

SVN commit 1061937 by mmalek:

Use Positive/Neutral/Negative color values from system color scheme instead of fixed green/yellow/red
CCBUG: 216812

 M  +7 -2      k3bjobprogressdialog.cpp  
 M  +8 -3      k3btempdirselectionwidget.cpp  
 M  +65 -65    misc/k3bimagewritingdialog.cpp  
 M  +17 -10    projects/k3bfillstatusdisplay.cpp  
 M  +10 -6     rip/k3baudioconvertingoptionwidget.cpp  
 M  +9 -4      rip/k3bvideocdrippingdialog.cpp  
 M  +10 -4     rip/videodvd/k3bvideodvdrippingwidget.cpp  


WebSVN link: http://websvn.kde.org/?view=rev&revision=1061937

a) Well, now when incorrect filesize is detected the image info area looks like this:

Detected: Iso9660 Image
Filesize: 340.1 MiB (different than declared volume size)    <- Negative color
Volume Size: 343.5 MiB
...

If that message gets ignored by user the following message is shown after clicking Burn: "This image has an invalid file size. If it has been downloaded make sure the download is complete.
Only continue if you know what you are doing."
What is not clear for a user at this point?

b) You're right, I have replaced all hardcoded red/yellow/green color values with corresponding Negative/Neutral/Positive system colors. Thanks, I didn't know such colors exists in system palette :)
Maybe I will also add a warning icons when I will be rewriting this image info area (it currently depends on KDE3Support, so it must be rewritten anyway). It will be done after 2.0 release, though.

ad.a) the message -- there is shown info about the "effect" not about the "cause" (the reason). It is wrong because:
a.i) we live in world of software bugs, without explanation user can assume the wrong is not the filesize but k3b
a.ii) rephrasing it in such way
"This image has an invalid file size"
-->
"The actual file size does not match the size declared in the file header."

Is longer I admit but contains all the info k3b has. It is also more accurate (how do you know the filesize is incorrect but not the header) -- here we give 100% correct explanation -- two values does not match, but we don't say which one (because we don't know). Please note the original message is not self-explanatory while the proposition is.

Of course I am all for shortening the message above but only if the information is preserved.

ad.b) you are welcome :-) Icons will be great addition! Thank you in advance for adding them.

SVN commit 1062018 by mmalek:

Changed the "invalid image file size" message to be more informative.
BUG: 216812

 M  +1 -1      k3bdirview.cpp  
 M  +5 -6      misc/k3bimagewritingdialog.cpp  
 M  +1 -1      option/k3bthemeoptiontab.cpp  


WebSVN link: http://websvn.kde.org/?view=rev&revision=1062018

I have changed the message to your version. Thanks for constructive feedback!