Hello,

1) For example when I write- Sale ends in: [countdwon timer shortcode]. It should display all that in one line with same font size and alignment like –
Sale ends in: 01 02 35 02

Header – it is not possible to put countdown close to the text. there two independent elements and there is always a gap between them http://prntscr.com/lcmu6e When you put the text and shortcode together in the same text block, the text is not visible.
In order to change the font size, use the code for .whb-main-header .woodmart-timer > span

Product page: Try to create the template with the help of columns instead of the Message box http://prntscr.com/lcmwto You can use separate icon and text. It will give you more flexibility

I have checked the code for the font size it works

Best Regards