help i need to know the size of each tile

That is a very good question.    

then you do this you should draw the rectangular floor - It will make it easier to follow. 

Let the tiles be x metres by x metres in size

9=c*x      and       12.3=k*x  

9/c=x       and        12.3/k=x   

where c and k are whole  numbers (cardinal numbers or positive integers if you like)

so      

$$\frac{9}{c}=\frac{12.3}{k}$$

$$\\\frac{9}{12.3}=\frac{c}{k}\\\\ \frac{c}{k}=\frac{9}{12.3}\\\\ \frac{c}{k}=\frac{90}{123}\\\\ \frac{c}{k}=\frac{30}{41}\\\\$$

this fraction cannot be simplified further.

So 30 tiles fit on the short side and 41 tiles fit along the longer side.

9/30=0.3 mtres           12.3/41=0.3metres

The tiles will be 30cm by 30cm.