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 Canon SX720 HS 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). Canon SX720 HS has external dimensions of 110 x 64 x 36 mm (4.33 x 2.52 x 1.42″) and weighs 270 g (0.60 lb / 9.52 oz) (including batteries).
Below you can see the front view size comparison of Nikon D3400 and Canon SX720 HS. Canon SX720 HS is clearly the smaller of the two cameras. Its body is 14mm narrower, 34mm shorter and 40mm thinner than Nikon D3400. As you can tell, actually this is not a fair size comparison since Canon SX720 HS 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 Canon SX720 HS.
Now lets look at the top view comparison of Nikon D3400 and Canon SX720 HS.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Canon SX720 HS is significantly lighter (125g ) 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 Canon PowerShot SX720 HS 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 and Canon SX720 HS's LCD screens has the same diagonal size of 3".
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.