Thanks to cristianyxtl and rondoan for their help. Found how to do it. You need to perform two operations:
1. Increase the size of spr_speedlimit_eu.svg along the path the_skin_by_pongo.zip\ ui_nextgen\res\common\nodpi\spr_speedlimit_eu.svg
2. Change the font size in params_override.css along the_skin_by_pongo.zip\ui_nextgen\common\ui\params_override.css (fontsize_speedlimit:58).58 is my choice.
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