If the size and the cost is a limiting factor for the embedded board selection, then you have this thumbnail size embedded board called Trinket from Adafruit. The listed price of this board in most of the online shopping portals is less than US$ 7.
In India it is available at a price of less than rupees 1400/- on Amazon India. The board is powered by Atmel's ATtiny85 microcontroller for processing, giving it 8K of flash memory and five available I/O pins. Can have the option to buy either 5.0 V or 3.3 V logic. 3.3V board is smaller than the 5V.
Be noted that two of the I/O pins are also used for USB communication. This port can be plugged into a computer and can be used as a human machine interface such as virtual keyboard.
Some of the key specifications include:
ATtiny85 on-board, 8K of flash, 512 byte of SRAM, 512 bytes of EEPROM
Internal oscillator runs at 8MHz, but can be doubled in software for 16MHz
Hardware I2C / SPI capability for breakout & sensor interfacing.
More on this board is available at: https://learn.adafruit.com/introducing-trinket/introduction