The shape itself is very important with the rectangular sleeping bags being more comfortable, as they allow you to move freely inside, but less effective. If you plan on spending the night at low temperatures, then you should probably make a few sacrifices in terms of comfort and choose the mummy sleeping bags. These are wider at the top and get narrow at the feet, which can pose a problem for these who were used to rectangular sleeping bags. On the other had the design is more efficient in keeping the heat inside and are recommended when temperatures are lower.
You wouldn’t like to sleep in sleeping bags that are constrictive, but you should avoid the ones that are too big for you as well, because they won’t succeed in warming you up. Just like when shopping for clothing, the size is just as important as the design, so if you intend of buying sleeping bags from the Kathmandu or other online stores, take your time and browse through the entire offer.