I've put together a light version of the Heatermeter using an Arduino Pro Mini (3.3 V, 8 MHz) connected to an HLK-RM04 (32 MB) as the wifi interface:
Size: 70x50x35 mm (2.8X2.0x1.4 inch)
Full Heatermeter 4.0 functionality except what is noted below
Full Linkmeter wifi interface including AVR...