In some journals, they ask that small figures should contain 300 words while large should have 600 words. So how we can count the words in a figure? secondly, If a figure contains 4 graphs like a,b,c,d, this will be a small or large figure?
I believe the word count for figures and tables is based on the amount of space the figure or table takes up in the final print version of the article. Find an article in the journal you are submitting to and see how much space a figure of similar size might take up on the PDF or printed page.
Here is an example of sizing figures (artwork) from Elsevier: https://www.elsevier.com/authors/author-schemas/artwork-and-media-instructions/artwork-sizing. Searching for author guidelines for your specific journal may give you more specific information.
Good luck on submission!