I'm planning to buy a 24-70 f/2.8 lens for my Canon 7D. I need a lens that won't increase its length while zooming (like the Canon 70-200mm series). Is there such a lens in any brand?
You need to look for a lens that uses internal zoom so that the length of the lens will not change during zooming. You also want a lens with internal focusing so that the length of the lens doesn't change during focusing.
Be forewarned, though. I'm not aware of any such 24-70mm lens made for use with a full frame or APS-C camera.
There are longer lenses with such features, such as the Canon 70-200mm series. These lenses use a telephoto design for their entire focal length range.
There are shorter lenses which come very close, such as the EF 17-40mm f/4 L and EF 16-35mm f/2.8 L II (the recessed front element moves very slightly when zoom is adjusted, but never protrudes beyond the front of the outer barrel of the lens). These lenses use a retrofocus design for their entire focal length range.
The difficulty with a lens such as a 24-70mm for DSLRs with a flange focal distance of between 40-50mm is that they often use a retrofocus design for their shorter focal lengths but as the lens is zoomed to the longer focal lengths they use intermediary internal lens groups that move to switch the optical formula to a telephoto design. For more about the design of these retrofocus/telephoto zoom lenses, please see: When you zoom in with a lens on an SLR why does the lens go in then out?
Short of building a lens with a flat plate at the front that leaves enough space for the movement of the front refractive group within the lens barrel, this is very difficult to do without changing the length of the lens. The disadvantage of the flat plate design is that it would extend the length of the lens at all focal lengths and thus add unneeded weight and complexity to the design at a cost of optical performance as well as increased manufacturing cost. So no one that I know of makes such a lens.
One interesting design is the original Canon EF 24-70mm f/2.8 L introduced in 2002 and replaced by the EF 24-70mm f/2.8 L II in 2012. The older design is fully extended at 24mm and fully retracted at 70mm. To the uninformed observer it appears that the lens is "zoomed in" when it is actually "zoomed out" and vice-versa.
One advantage of this unusual design is that the lens hood can be attached to the main barrel and doesn't move at all as the lens is zoomed. When the lens is extended at 24mm it is close enough to the front of the hood to prevent vignetting at that wider angle of view. When the lens is retracted at 70mm the front element of the lens is recessed further back in the hood which provides a narrow cone of shading that is more optimized for the narrower angle of view.
There is no such lens. Internal zoom lenses generally have a shorter zoom-ratio due to the fact that they do the zoom by moving internal elements. few telephoto lenses take exception to this.
The closest lenses to your request would be Sigma A 24-35mm F/2 which is available in Canon EF mount. Canon has a few but they are much wider such as the 11-24mm F/4L or 17-40mm F/4L.
I don't know if any manufacturer make 24-70mm internal zoom lens. Another strategy is to buy a prime 35 or 50mm or 60mm macro and use your feet to move backwards or forwards. Also, in long run prime lens will help reduce dust accumulating on the sensor and also tend to have better image quality.