Sizes are Width x Depth, and based on the visible size on the lot - they may be slightly larger or smaller as far as usable footprint goes as this didn't show in-game so I just did my best counting. These should at least give you a starting point in planning your lot sizes for your new neighborhoods in Create a World.

City Hall: 47x20
City Hall (Riverview): 31x20

School: 34x20
School (Riverview) 34x20

Hospital: 33x25
Hospital (Riverview): 33x25

Bookstore: 10x13

Business Office: 21x21
Business Office (Riverview): 21x21

Mausoleum: 6x8
Mausoleum (Egyptian): 6x8
Mausoleum (Chinese): 6x8

Military Base: 39x39
Military Base (Riverview) 39x39

Supermarket: 12x15

Diner: 18x9

Bistro: 13x14

Stadium: 48x31
Stadium Sign: 7x2

Science Dome: 51x58
Science Building: 35x20
Science Building (Riverview): 35x20

Criminal Warehouse: 20x22

Police Station: 18x18

Theater: 16x26

Day Spa: 14x12

Radio Tower: 18x19
Satellite Dish: 12x10
Water Tower: 19x18
Water Tower (Riverview): 19x18