What clouds appear long, flat, and spread across the sky?

You're describing stratiform clouds, but in order to provide a more thorough response, we need more details.

A cloud is classified as stratus if it is uniform and lies low to the ground, or stratocumulous if it is made up of different elements (darker and lighter areas) and lies relatively low to the ground.

Altostratus refers to a uniform, mid-level cloud, while altocumulous refers to a mid-level cloud composed of elements.

It is cirrostratus if the cloud is uniformly high in the sky.

The fact that your question states "spread across the sky" means that it is not circular because it does not encompass the entire sky.

Given the length of your question and the seeming implication of elements, I would assume it is either stratocumulous or altocumulous.

If the cloud was translucent, that would be useful to know.

The clouds that appear long, flat, and spread across the sky are called stratus clouds.