In the display canvas, the granularity of selections is based on pixel width, which depends on the length of the file that is loaded. You can stretch Audio Tool's base window to see more detail when you are selecting segments.
You may find it useful to use multiple Audio Tools. For example, you can edit a small region of a large file by selecting a segment that is somewhat larger than the region and copying the segment to another Audio Tool display canvas. Then, you can easily edit the segment in the other Audio Tool by stretching the tool to provide better pixel resolution. When you're finished editing, you can copy the edited region back into the original Audio Tool.
You can also use multiple Audio Tools to assemble an audio track from multiple sources.