It depends if you have enough wood to last the whole night. If you are warming yourself just before sleeping with no intentions to maintain the fire all night, you'll want to keep the opening small. If you are planning to burn all night, and have a big pile of wood, you'll want to made the opening wider rather than taller to better trap the heat without losing so much. Also so you can crawl out easier if it catches fire

It is also important to make some sort of backstop for the fire to better reflect heat into the shelter using either stones or earth covered logs.

Or make it big enough to put the fire inside depending on what your building materials are.

Regards,

Tom