In version 24.12.1 (standalone installation under Windows 11), I define a subtitle style with a huge font. It shows fine in the preview of the style. However, the subtitles with the style assigned still look tiny. In fact, nothing I do in the style seems to matter.
Most probably because you’re not using that style with the subtitles you see not changing. ASS allows subtitles to have and use multiple styles. If you want clips to use your new style you need to change them to use it. Likewise if you want to change the style options of existing clips using the “Default” (or any other) style, you need to change its parameters.
Simply adding a new style will not change them at all - that just gives you a new style with the option to use it for some or all clips in place of the previously existing ones.