Smaller tiles?

No, no, not tile size.

I mean, the tiles are a fixed size on-screen, no matter what dimensions you use at the import, right?
And is there any way to change that size?

You can create a tileset using any size tiles you want (you specify when importing the image).  You can also specify the tile size to use when you create a new scene.  If these match, it should work fine.

I think what he means is how large/small they appear while playing.

Flash uses pixels for all measurements. Thus, if a tile is 32 pixels wide it will always be 32 pixels wide in the game. If you draw a character that is supposed to be as wide as one tile, but you draw him 64 pixels wide, he will fit over the top of two 32px wide tiles.

There is no way to grow/shrink the tiles other than in an external art program (and then re-importing it at the new size).

However! If the problem is that your game is too small, such as a retro styled game with small sprites, you CAN set the zoom level of the Game Properties to increase the size of everything to make it easier to view on high-resolution screens when playing.

I think you didn't get the problem.
I imported tiles with 8*8 sprites, and it shows a magnified 32*32 pixels in game, while the player is still the original 8*8.

Either your tileset or scene is set to use 32x32 tiles. There's a button to change the tileset size in the tileset editor, but you can only define scene tile size when creating a new scene.