How can I define suitable value for big objects

Hi all in this image, I want to remove big objects. I use this code

BW2 = bwareaopen(thresh_image_conv,5000); newBW2=imsubtract(thresh_image_conv,BW2);

So is there a way to define appropriate value for this kind of objects. Something like a tolerance should be. Because i found this value emprically. The original and selcted ones are attached there. You can see red ones are selected big. But there should be more acording to my code. This level is belong to me so how can i make a decision automatically this value. Thanks for suggestion.