Thanks for getting back with a link.
The slider changes its height because it follows the height of whatever image you put in there. However, its width is always 100% of its container because this is the nature of responsive images. If you resize the width to a smaller size, some of the content gets cut off. You can try this out for yourself:
.flexslider {
max-width: 80%;
}
You could try adding another element, adjust its size and then insert the slider inside this element. I’m not too sure how comfortable you are with WordPress template files, but unless you’re a developer, my recommendation is to keep the slider width at its default size.
As for the header height, you could adjust the height using this code:
.site-header {
max-height: 200px;
}
Replace 200px with your chosen height value. However, be mindful of the fact that if you make the header a fixed height instead of letting it grow with its content, the content could spill over, especially on smaller screens.
To change the font size in the site title, use this CSS:
.site-title {
font-size: 40px;
}
And to change the tagline, use this snippet:
.site-description {
font-size: 20px;
}
You’re free to replace 40px and 20px with your chosen font size.
I hope this helps ??