It seems that for the decorations, it doesn't matter often times if you get a 1x1 or a 2x2 as far as..

You want to increase your flower density. The highest flower density available is 2400 flowers in a 2x2 decoration (i.e. Fire Engine, Grand Greenhouse, Park Playtime), which equates to 600 flowers per 1 space! There is no 1x1 decoration that even comes close. You'll be lucky to get over 100 per space (imagine the space 24 of these would take, compared to 4 spaces with 2400 flowers!)

If those 3 super decorations are not available to you, yet, look for other high flower items, the higher the better. Intermix with 1x1 greater than 100 flowers and you'll be sitting pretty. And always check out the flower density in the seasonal set, which may be better than what you can get in the regular store (and faster, too), depending on your level.