@aliphpdeveloper,
Sorry, we were unable to reproduce the issue regarding change in the size of indexLabel at our end. Can you please create a jsfiddle reproducing the issue and also share browser and OS details along with versions where you are facing this issue.
Regarding your query about placing indexLabels at the bottom, this feature is not available out of the box as of now. However you can follow this workaround.
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Suyash Singh
Team CanvasJS