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 Nikon D3400 and Nikon S2900 side-by-side from the front, back and top in their relative dimensions. Nikon D3400 has external dimensions of 124 x 98 x 76 mm (4.88 x 3.86 x 2.99″) and weighs 395 g (0.87 lb / 13.93 oz) (including batteries). Nikon S2900 has external dimensions of 95 x 59 x 20 mm (3.74 x 2.32 x 0.79″) and weighs 119 g (0.26 lb / 4.20 oz) (including batteries).
Below you can see the front view size comparison of Nikon D3400 and Nikon S2900. Nikon S2900 is clearly the smaller of the two cameras. Its body is 29mm narrower, 39mm shorter and 56mm thinner than Nikon D3400. As you can tell, actually this is not a fair size comparison since Nikon S2900 has a built-in lens but Nikon D3400 doesn't. Once you put a lens on D3400, its thickness will increase significantly.
Here is the back view size comparison of Nikon D3400 and Nikon S2900.
Now lets look at the top view comparison of Nikon D3400 and Nikon S2900.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon S2900 is significantly lighter (276g ) than the Nikon D3400 which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on D3400, the total kit will weigh much higher.
Unfortunately neither Nikon D3400 nor Nikon Coolpix S2900 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 Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Nikon D3400's 3.00" LCD screen is slightly larger than Nikon S2900's 2.7" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.