Hello, thank you for participating in AskMath! I'm glad to help, but I'd advise you to use proper punctuation for your question, the text is confusing as it is. My understanding is that you want to fill the wall with perfectly square mirrors.

In brief, a good mirror size would be 26 cm. That way you can put 3 mirrors horizontally and 7 mirrors vertically, a total of 21 mirrors.

I think you would like to use more mirrors vertically because the wall is taller than it is wide. The ratio of these sizes is 205/88 = 2.33, which is roughly 7/3. So it would be interesting to use 7 mirrors vertically and 3 mirrors horizontally because the number of mirrors would try to match the shape of the wall.

To find the length of the mirror we have to sum the size of each mirror and the spacing between them, note that for 3 mirrors there is 4 spacings. let's call the mirror length L and the spacing lenth T.

3*L+4*T = 88 cm --> L = [ 88 - 4*T]/3

Assuming a spacing of 2 cm, the mirror size should be 26,67 cm, and a spacing of 3 cm is a mirror size of 3 cm --> 25,33 cm. So a mirror size between these values would give you a similar spacing. I choose 26 cm to get a nice round number. One important thing notice is that because we approximated the fraction at the start of the problem, the horizontal and vertical spacing will not be exactly the same, but should be close.

Feel free to ask if you have any further questions