The Teensyduino download is 22 megabytes for the Windows version, 30 megs for Mac, 24 megs for Linux. (full disclaimer: I’m the author of Teensyduino!) Those sizes are for version 1.13. Like all software, the size will likely change over time as more capabilities are added.
Teensy runs very similarly to regular Arduino boards. Your code runs directly on the hardware with true real time access, just like Arduino… except the ARM processor is faster than AVR and your code is 32 bits, so using “byte” and “long” are the same speed. All the normal functions like digitalRead, pinMode, analogRead, analogWrite, pulseIn are supported. Many of the widely used Arduino libraries are supported (and numerous others also work). Here’s the official library list for Teensyduino:
http://www.pjrc.com/teensy/td_libs.html
Teensy does NOT run Linux like Raspberry Pi. If you give it a try, you’ll discover it works very much like Arduino. Probably the biggest difference you’ll notice is the Upload process is much faster!