Size and weight are big decision factors when you are trying to find the ideal camera for your needs.
In this section, we are going to illustrate the Canon 100D and Canon 1200D side-by-side from the front, back and top in their relative dimensions. Canon 100D has external dimensions of 117 x 91 x 69 mm (4.61 x 3.58 x 2.72″) and weighs 407 g (0.90 lb / 14.36 oz) (including batteries). Canon 1200D has external dimensions of 130 x 100 x 78 mm (5.12 x 3.94 x 3.07″) and weighs 480 g (1.06 lb / 16.93 oz) (including batteries).
Below you can see the front-view size comparison of the Canon 100D and the Canon 1200D. Canon 100D is clearly the smaller of the two cameras. Its body is 13mm narrower, 9mm shorter and 9mm thinner than Canon 1200D.
Here is the back view size comparison of the Canon 100D and Canon 1200D.
Now lets look at the top view comparison of Canon 100D and Canon 1200D.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Canon 100D is significantly lighter (73g ) than the Canon 1200D which may become a big advantage especially on long walking trips.
Also keep in mind that body weight is not the only deciding factor when comparing two interchangeable camera bodies, you also have to take into account the lenses that you will be using with these bodies. Since both Canon 100D and Canon 1200D 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.
Weather Sealing
Unfortunately neither the Canon EOS 100D (EOS Rebel SL1) nor Canon EOS 1200D (EOS Rebel T5 / EOS Kiss X70) provides any type of weather sealing in their bodies, 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 100D and Canon 1200D's LCD screens have the same diagonal size of 3.00 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.