we engineering students are preparing for a particular competition with dimensions of 170 in. (length + breadth + width) for that competition we require landing gear but for that are we supposed to use suspension in it?
For one that size, you'll certainly need some spring in the landing gear, but it's likely that shock absorbers are still unneeded. Most models use spring wire landing gear legs, at least up to around 5 kg weight. As big as your model will be, you may need hinged gear legs with spring return, or something similar.
I'd suggest looking at the landing gear for smaller crewed aircraft (like a Cessna 152 or Piper Tomahawk). The smaller Cessnas for certain have only spring legs for the main gear, though they use a shock strut for the nose. Scale down from their practice and you should be good (though scaling down frequently means simplifying, as well as making smaller).