Room size and grid

Fairly new to roll20 dming so sorry for the noob question.I am running rise of the runelords and i have uploaded a map and on the thistletop dungeon and as i am adding creatures into rooms i am starting to wonder for some of the rooms its sized as 10x10 with one npc in it already. if you look at the grid 3 more creatures can fit in the room.is that correct? and i assume that takes beds and tables into consideration?

Without seeing the actual rotr map, it's hard to tell...  Is there a grid on the original map?  If you are using roll20's grid overlay, you can adjust the scale from the page's settings.

If the room is 10x10, and your Roll20 Page Settings has Scale: 1 unit = 5 feet, and Grid Enabled with Grid Size "1"... then the 10x10 room would (should, correctly) have 4 squares. It sounds like that's what you described. It is possible someone might have to be standing on a table or a bed. If I misunderstand the question, can you post a screenshot of your page settings or describe it again?

The grid space a token occupies doesn't necessarily mean that the creature takes up the entire 25 square feet.  It means that that creature controls that much space.  But yes, if you have a 10x10 room with one creature in it, there's only room for 3 more medium/small size creatures.