Holding down the alt key and moving your mouse up will make the item max size.
Check your "Placement Mode" when trying to put items inside other items. Free Placement Mode is the only one that allows for free reign to do so. Cursor mode will allow it only up to a point. Collision mode won't work for it at all. Wall placement mode is generally used only when actually putting things on the wall. Some items will not go into free placement mode. I'm looking at you, fountains and houses!
When you have your particle effects (sparklies!) turned on, the boxes of fireflies, ect have graphics. The box of butterflies has butterflies that fly around. The fireflies has fireflies flying around, cloud has clouds, ect. The larger the box, the bigger of an area these sparklies are in. Always a good idea to turn your sparklies on when doing housing (or touring housing!) so you can see all the cool effects. Nearly all mage summoned pet weapons have sparklies of some sort on them as well, and many of us use those sparklies for their own effects. You may be missing a lot of the intended viewing experience, with them turned off.
Another note on the boxes.... you should sink them just under the ground, when you place them, or tuck them into a structure - furniture, wall, tree, SOMETHING - so you don't have a box just sitting there looking kinda goofy. Just size it how you want it, and tuck it into place.
To make an object appear to float, like a house or other structure, place an item (or items) to the appropriate height. Doesn't really matter what they are. Place your house or structure that you couldn't get to float on it's own on top of the tower-thing you just made. Once it's placed, remove the items under it. It will leave your item suspended, with nothing underneath it.
Hope this answered some of your questions, and maybe helped you out some!
EDIT: Just thought I'd also mention, items like fountains, which have no free placement mode, can sometimes be used in ways that make it appear that they do. I use them as sinks in my kitchen a lot, by lowering the counter down, then placing the fountain, then raising the counter back up. Just because I can't sink the fountain itself, doesn't mean I can't have a fountain sunk into a counter!