The Nano Every is a direct replacement for the Arduino Nano. It runs on 5V and is the same size as a Nano, 45 x18mm. However it is a more powerful board than a standard Nano or Uno. In particular:
Uses the ATmega4809 instead of the ATmega328
Has 50% More program memory (you can use WiFi or Ethernet!)
Has 200% More flash memory
Micro-USB connector instead of Mini-USB
Clock 16 MHz *
Flash Memory 48 KB
SRAM 6 KB
EEPROM 256 Byte
Analog Input pins 8 (with 10-bit resolution)
Digital IO Pins 23
PWM pins 6
no crystal, internal clock. Not sure about steady interrupts. And despite advertising 20MHz, it runs instead at 16.
5V regulator can handle much more power (1A theoretical), 3.3V 600mA. 4 UARTS! But serial1 is wired differently than on the Nano. Where pins 0 and 1 on a Nano are serial and connected to the USB and pins 0 and 1, the Every runs serial 1 to those pins.