Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Canon 600D and Canon 550D side-by-side from the front, back and top in their relative dimensions. Canon 600D has external dimensions of 133 x 100 x 80 mm (5.24 x 3.94 x 3.15″) and weighs 570 g (1.26 lb / 20.11 oz) (including batteries). Canon 550D has external dimensions of 129 x 98 x 62 mm (5.08 x 3.86 x 2.44″) and weighs 530 g (1.17 lb / 18.70 oz) (including batteries).
Below you can see the front view size comparison of Canon 600D and Canon 550D. Canon 550D is clearly the smaller of the two cameras. Its body is 4mm narrower, 2mm shorter and 18mm thinner than Canon 600D.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Canon 550D is 40g lighter than the Canon 600D but we don't think this will make a significant difference.
Also keep in mind that body weight is not the only deciding factor when comparing two interchangeable camera bodies, you have to also take into account the lenses that you will be using with these bodies. Since both Canon 600D and Canon 550D have the same Canon EF/EF-S lens mount and APS-C sized sensors, lenses will not be a differentiator on total size of the system.
Unfortunately neither Canon EOS 600D (EOS Rebel T3i / EOS Kiss X5) nor Canon EOS 550D (EOS Rebel T2i / EOS Kiss X4) provides any type of weather sealing in their body, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Canon 600D and Canon 550D's LCD screens has the same diagonal size of 3".
On the other hand, Canon 600D sports a Fully Articulated screen
which you can change the angle and position of the screen almost freely, making it possible to shoot from waist or over the head levels, and especially very useful for shooting selfies and video. Canon 550D's LCD screen is a fixed type which provides almost no flexibility in shooting positions compared to 600D.