Hi Dan -- It sounds like you have a nice old penny, half penny, or farthing from Great Britain. These coins have similar designs and the denomination is based on size. Since you indicate a 'large' coin, you probably have the penny. Here are the dimensions:
FARTHING: 20 mm diameter
HALF PENNY: 28 mm
PENNY: 34 mm
They all have King George III on the front and Britannia on the back.
There are not many of these still around in decent shape. The one in the picture, a penny, might sell for $10 US dollars as it is average circulated condition. Since yours is worn, figure less value. Here are some approximate catalog values for the three denominations in average circulated condition with no spots, scratches, stains, nicks or cleanings:
worn: $10 US dollars approximate catalog value
average circulated: $30
well preserved: $120
fully uncirculated: $300
farthings dated after 1798 are more common and worth half these values
worn: $15 US dollars approximate catalog value
average circulated: $50
well preserved: $250
fully uncirculated: $700
1/2 pennies dated after 1798 are common and worth one-third these values
there is a 1/2 penny dated 1772 with GEORGIVS spelled GEORIVS, these are worth less than the values above
PENNY (dates 1806 and 1807 only):
worn: $6 US dollars approximate catalog value
average circulated: $18
well preserved: $150
fully uncirculated: $550
PENNY (date 1797 'cartwheel):
This coin has a different design. You can see our appraisal page for the 1797 'cartwheel' penny at this CoinQuest link.
Be sure to use our Terminology page to interpret catalog values.
You can also check one of our favorite web sites on old British coppers: AboutFarthings.co.uk.