It's a 4" x 4" card. I like this size as it's suitable for a quick note or a full card, especially for people who you might want to give a card to but you don't necessarily know that well.
As you can see, I've kept it quite simple and this would be a great design to mass produce, also, a quick change of the DSP and colours and you've got a card suitable for men too.
The main image is the cake from the "Sale-a-bration" exclusive set called "Patterned Occasions" and I've used "Tea for Two" DSP, I think they go together rather lovely. I've also put a small spot of "Crystal Effects" on the love heart, so, in real life, it pops nicely off the page.