Limitations of Scratch

Title maximum length: 100 characters
project.json uncompressed file size limit: 5MB
Individual asset (costume or sound) file size limit: 10MB
There is no limit on sb3 file size.
There is no limit on block count but due to the project.json size limit, expect that only 20000-30000 blocks can fit within it.
Project framerate: approx. 30FPS

Sprites are visually snapped to a grid sized with 480x360 steps on the screen.
Size is limited by the current costume, when scaled it must not be visually bigger than 150% the size of the stage (720x540) on any axis or smaller than 5 pixels on any axis. Source code
Maximum number of clones at one time: 300 (limit shared by all sprites)
Sprites can not be positioned off-screen with a margin less than 15px still showing. Source code

Precision is limited to 5 bits red, 5 bits green, 4 bits blue. (this is only 14-bit color, much less than 24-bit RGB that is displayed by Scratch). Source code

Supported data types: booleans, double floats, strings. Through hacks it's possible to utilise a few other types such as arrays.
List item limit: 200000
There is no Scratch-imposed limit on string character count in variables and list items.

10 variables per project
Setting the value of any cloud variable has a shared average rate limit of 10 times per second.
Variables may either store a floating point number (NaN and Infinity excluded) or a string containing numerical digits 0-9, up to a length of 256 characters.
Calculated from the above limits, the maximum data transfer rate is approximately 1063 bytes/sec.
Users with "New Scratcher" status can not set cloud variables with values that other users can see.