<p>My 3D prints shift along the Y-axis on my Ender 3 3D printer. I don't know what to do. My Y-axis belt is tight, So I don't think that is the problem...</p>
<p><a href="https://i.stack.imgur.com/zVtPS.png" rel="nofollow noreferrer" title="Y-axis layer shifting"><img src="https://i.stack.imgur.com/zVtPS.png" alt="Y-axis layer shifting" title="Y-axis layer shifting" /></a></p>

8 mm rods and 6 mm GT2 belts are generally accepted as a good tradeoff between price and performance, an exact calculation is possible but might not be very relevant if another part is flexing. Also, generally speaking, the smaller the part the sooner it will wear out of specification. Thus your service interval might be higher compared to an over-engineered printer.
In short, it depends on what your goal is, if you desire low maintenance and accurate machine, you might be better off with heavier gauge parts. Obviously, this will also affect the speed of printing.
A 6 mm GT2 belt might have a higher stretch factor compared to a 10 mm belt, but can be mitigated by adjusting the acceleration. In addition, a 10 mm belt has a larger pulley reducing the number of steps per mm, lowering precision. As such you might be better of using two 6 mm belts.
Increased rod size for the print bed will not affect printing speed much but might help with accuracy since the bending modulus is lower. Play around with the calculators below to get an idea of the force your beam will have to withstand. That said, there are a lot of other factors that will flex under load, for example, the bed leveling springs. You can replace them with solid spacers, but that might warp the bed when it heats up.
https://www.engineering.com/calculators/beams.htm
https://www.omnicalculator.com/physics/acceleration
To conclude, I would use the calculators to figure out if the 8 mm rods are within tolerance for the intended speeds and load, but don't forget to look at the overall picture. The quality of parts you choose is one such thing.