what is the correct chain size for my bike
You need an 8 speed chain that is 3/32" wide.  This is a "normal" bicycle chain.  Often sold as a 6/7/8 speed.  This chain will be somewhere between 112 and 118 links long.

You also need the old chain to figure out how many links it had.  Lay them out side by side, and notice the old chain will appear to have longer links due to wear.  Figure out which pin on the new chain lines up with the **same-number* of links on the old chain.  Then use a chain breaker tool to mostly-remove a pin and take off the excess links.

You might prefer to use a quicklink to rejoin the chain, or refit the pin that is mostly-out.

Do not bother getting a 9, 10 or 11 speed chain.  Its a waste of money for an 8 speed bike cassette.

However depending on the wear of your cassette, it may be worth replacing that too.  Old chains and old cassette wear together, and just replacing the chain may give you chainskip.  If that happens, time for a new 8 speed cassette.