How to change the size of textbox in proportion to the size of header cell on changing header cell size.

Built-in resize functionality always preserver header and body parts of grid in sync, so the size of columns always autoaligned
If by some chance you was able to de-sync header and body parts of grid, it always may be restored by next command

If problem still occurs for you - please provide any kind of sample where error can be reconstructed ( you can send it directly to support@dhtmlx.com )