Different cells have different sizes. Given are the following cells:
I. Mycoplasma 
II. Ostrich eggs
III. Human RBC
IV. Bacteria
The correct ascending order of their size of the cells given above will be : 
1.  I, IV, III, II   
2.  I, I, IIL, IV
3.  II, I, III, IV 
4.  III, II, I, IV

(a) Represents the correct ascending order of cell's size.
   Cells in organism vary greatly in their size, shapes and activities.
I.   Mycoplasmasare the smallest cell with size only 0.3 mm.
ll.  Bacterial cell are of size 30-5um
llI. In human red blood cells are about 7.0 um in diameter.
IV. Ostrich eggs are among the largest cells with size (15x 13)cm.
Mycoplasma Like Organisms (MLOs) or mycoplasmaare the smallest cell followed by the size of bacterial cell, then RBCs and ostrich egg cell is the largest known cell.
Page 126-127, XI NCERT