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Title and the picture say all..

ACP > Templates > Your theme's templates > Forum Display Templates > forumdisplay_thread > and find;

{$thread['subject']}

and Change it into;

<span style="font-size: 13px;">{$thread['subject']}</span>

Change 13px to whatever size you would like to use

Thank you. One more question, and if I want to change to display colour?

Following the same steps above, replace it with:

<span style="font-size: 13px; color: #000;">{$thread['subject']}</span>

Change #000 to whatever color you want (e.g. #fff for white, #ff0000 for red).

I tried this method already but it dont work
EDIT: My bad,now is working :d. Ty