Open WED and select your floor block. You can now see the block properties on the left-hand side. Click on the register Object and check the "Bound" values. I assume that your block simply is way too big. As a reference, my player is usually between 90 and 190 quants high, some people even prefer smaller scales.
Additionally, try to click "Default Settings" in the Map Builder dialog.
If you do not need shadows and/ or want so save some video space, you can set the Flat shading texture option, either in the texture window -> rightclick -> settings before you assign the texture, or select your block and set the surfaces manually to flat.