here's a working version:

1. float radius;
   int timespressed,last;
   
   void setup()
   {
     size(300,300);
     radius=10;
     smooth();
   }
   
   void draw ()
   {
     background(0);
     ellipse(width/2,height/2,radius,radius);
     radius++;
     radius%=width/2;
   }
   
   void keyPressed()
   {
     if(key== ENTER)
     {
       timespressed++;
       if(timespressed>last){
         radius=0;
         last=timespressed;
         timespressed=0;
       }
     }
   }
   

if you'd like we can discuss why your version didnt work...

some thoughts about coding in general:

first of all you may want to logically plan your sketch before doing any coding. i use pieces of paper for that, and it really helps. e.g. here you obviously needed a variable to store the minimum number of presses to zero the circle, (thats why i created 'last'), which changes at every successful zeroing, and this requirement would have been evident if you had a plan on paper. by a plan i mean stuff like this:

If A happens then B should happen.
Find a way to see if A happened or not?
A happens, when x is greater than y.
Make variables to store x and y.

so on...(this could also be done by drawing logical connections and stuff, it really depends on how your mind works)

syntax (coding) problems with your sketch:
 "int timespressed = ENTER;" - doesnt make much sense. before writing a line of code you should always think about the purpose of that line. (im sure you had something in your mind when you wrote this, but i just cant figure what :).)

well, i hope this helps

and here's a commented version of the working version, each comment states the purpose of that line, it may teach you something:

1. float radius; //make variable called "radius", which stores float type of numbers
   
2. int timespressed,last; //make two int type variables, called "timespressed" and "last" (this could have been done in two separate lines, its just shorter)
   
3. void setup() //tell the program what to do once after pressing run
   
4. {
5.   size(300,300); //set the size of the sketch
   
6.   radius=10; //assign an initial value to sketch
   
7.   smooth(); //enable antialiasing
   
8. }
9. void draw () //tell the program what to do in every cycle (over and over again) after setup has been ran once
   
10. {
    
11.   background(0); //set the background black
    
12.   ellipse(width/2,height/2,radius,radius); //draw an ellipse at the center of the screen, using the variable 'radius' as... well, not radius, but diameter, take a closer look at the ellipse() function
    
13.   radius++; //increment 'radius' by one (the circle will be one pixel greater in the next cycle)
    
14.   radius%=width/2; //this is a bit tricky, i dont know if you are familiar with '%' (no, its not percent) or not, if not then click, also 'x%=y' is a shorter way of saying 'x=x%y', (it also works with other mathematical operations, like 'x+=y' means 'x=x+y' etc...)
    
15. }
16. 
    
17. void keyPressed() //what to do when a key (any key!) is pressed
    
18. {
19.   if(key== ENTER) //check whether the pressed key is ENTER, if yes, continue
    
20.   {
21.     timespressed++; //increment 'timespressed' by one (since we pressed ENTER)
    
22.     if(timespressed>last){ //check whether it was pressed more times than the last time, if yes, continue
    
23.       radius=0; //zero the circle
    
24.       last=timespressed; //store ("make the program remember") how many presses were needed this time to zero the circle
    
25.       timespressed=0; //start a new counting of presses
    
26.     }
27.   }
28. }

also, the reference (an encyclopedia-like collection of every single word and expression and statement processing understands) is a real eden

PS.: Try and find more significant subjects, 90% at least of the topics is to seek for help, so if each and every one has the same subject, we are in trouble to find a specific one!