When it comes to sizes of elements to use on my layouts, pairing 6 x 4" with 3 x 4" is one of my favorites to work with. You can swap between the two sizes easily and they can be arranged together seamlessly. Throw in some cards of the same size and you've got a layout that can come together in record time.
For this layout I used one 6 x 4" photo with three 3 x 4" photos and two 3 x 2" photos. Then I filled in the rest of the design with 3 x 4" cards. If you look at each side as an individual layout, all I've done is taken the design on one side and flipped the elements - photos on the bottom/cards on top on the left side, photos on top/cards on bottom for the right side.
With the cards, I used three on each side of the layout. On the left page I used one as a background for my title.
On the right page I used one as a background for my journaling.
Then I added a little bit of stitching to create a border in the corners with the cards.
This layout design as a whole is a concept I like to play around with often. There are so many different combinations of 6 x 4" and 3 x 4" elements you can play around with. It's easy to add more photos or less photos or customize to fit different themes. You can even extend it to the left and right edges or the top and bottom just by adding a few more photos or cards.
If I'm in a struggling with a design, this is definitely a go-to!