If you’re precomposing multiple layers, the precomposition will have the dimensions of the original composition. If you’re precomposing only one layer, you can choose whether to use the dimensions of thelayer or the composition.
If you’d like to have more options, I suggest making a feature request.
Here’s a description of precomposition options from “Precompose layers”:
Leave All Attributes In
Leaves the properties and keyframes of the precomposed layer in the original composition, applied to the new layer that represents the precomposition. The frame size of the new composition is the same as the size of the selected layer. This option is not available when you select more than one layer, a text layer, or a shape layer.
Move All Attributes Into The New Composition
Moves the properties and keyframes of the precomposed layers one level further from the root composition in the composition hierarchy. When you use this option, changes you applied to the properties of the layers remain with the individual layers within the precomposition. The frame size of the new composition is the same as the frame size of the original composition.
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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