Just point to the first element of your list of articles using CSS pseudo class like this:
PS. I’m so used to less that I’m gonna give you the .less syntax, but you can figure it out on your own right?
.container-with-your-posts {
> .post-container {
&:first-child {
h1 {
your styles for the title
}
img {
your styles for the thumb
}
}
}
}
This will work in IE8 if you declare the doctype. If you don’t care about the IE8 (and you really shouldn’t) you can also use :nth-child(1)
Edit:
Ok I can see in the comment below you are struggling with less so here you go – I’ll use CSS. Since you want to have this on archives we start with that:
.archive {
}
Then let’s select the posts container:
.archive #content {
}
Now let’s select all articles (posts):
.archive #content article {
}
And narrow it down to the first one:
.archive #content article:first-child {
}
Now to be sure we will not point other articles in the content (unlikely, but still – better safe than sorry) let’s add a direct child selector. As said it’s a bit overkill but what the heck:
.archive #content > article:first-child {
}
So now you are in the first article (post) in your archives list. Now you can narrow it down as precise as you want (going one selector at a time) or just from here you can go like this for the thumbnail:
.archive #content > article:first-child .entry-thumbnail {
width: //set your width;
}
and like this for the title:
.archive #content > article:first-child .entry-title {
font-size: //set your size;
}