For nail down wood flooring, an installer needs to provide for expansion in the width...not so much for length, although it is good practice not to fit net to any vertical surface.

Solid strip hardwood flooring has less than 0.2% movement parallel to the grain.

Every joint between the strips opens and closes as the wood shrinks and swells with moisture content.

Only one side of each strip is actually nailed, allowing each piece to move separately.

Non 'tongue and groove' wood that is face nailed can be a problem.
The wood can split at the nails since it cannot move.

'Engineered' wood flooring is often installed so it moves as a single large pieces of wood in every direction (especially if it has cross ply type construction).

Clearance must be provided in both directions, and for a large are can quickly mount up to inches of movement.

Good advice above additionlly for areas wider than 20 feet it is recommended that you start in the middle of the room and face the two boards groove to groove joined by a spline which will limit the expansion and shrinkage with areas wider than 20'. As stated above there is not enough expansion in the length to really worry about 'BUT guidelines state we will leave a 3/4" gap for 3/4" thick hardwood.