You can use this CSS selector: https://css-tricks.com/almanac/selectors/n/nth-of-type/
So you could do something like this possible:
.blog .type-post:nth-of-type(1) {
width: 100%;
}
.blog .type-post:nth-of-type(2),
.blog .type-post:nth-of-type(3),
.blog .type-post:nth-of-type(4),
.blog .type-post:nth-of-type(5),
.blog .type-post:nth-of-type(6),
.blog .type-post:nth-of-type(7) {
width: 50%;
float: left;
}
And so on – it will require some tweaking, but that should get you on your way 🙂