[useSize] Investigate ResizeObserver pooling

## Motivation

Similar to how [react-intersection-observer](https://github.com/thebuilder/react-intersection-observer) handles registering observer callbacks, construction of the `ResizeObserver` object may be pooled.

Start by locating the useSize implementation and compare its ResizeObserver registration with the pooling approach in react-intersection-observer. The issue does not define acceptance criteria; completion would require determining whether pooling is appropriate and agreeing on the resulting scope.

Written by the indexing model from the issue text.