How to flag a cell a colour based on measurements entered elsewhere?

I have a spreadsheet which calculates cost of lots of different types of materials based on the size entered.

there is a length cell (B5)
and a width cell (B6)

these are there for the user to enter the size into in mm.

the table below that has lots of types of materials with rows of prices which change based on the size entered and each column represents what the material will be applied to.
(I have attached a screenshot of it)

for an example i will use row 13 which represents Vinyl 610mm
there is column for media only, and the vinyl applied to various types of board 3mm/5mm/10mm etc, the formula in each cell calculates the cost based not he measurements and base costs worked out on a separate tab of the spreadsheet.

how can i additionally tell specific cells to highlight a colour if the size entered is larger than the media to warn the user?
For example cell B13 is the Vinyl, and shows the cost for the vinyl only at the size entered, but the maximum width for this is 610mm so i want it to highlight red to warn the user that the size entered is not possible.

Anyone able to help? if you need more info or a clearer explanation let me know, keen to try and get this sorted. cheers!

excel file attached let me know if my request needs any clarification

Thanks for your response, yes the price calculated isn't necessarily wrong in those fields but those are narrower rolls of media, but id like to say for those cells if width is greater than 610 then turn red, and in the other if it is greater than 1220 then turn red etc, i just don't know how to set the conditional formatting to do that, if you could give me a quick explanation how to set that id appreciate it.

I'm sure its simple but I'm not 100% sure where to start, its a feature I've not used before.

think i figured it out, went to conditional format> +rule> classic> use formula and added as you suggested =B6>610 then chose the highlight red option....

this seemed to work, however i wanted to do this for multiple different cells instead od setting them individually, but if i try selecting multiple cells and setting this the same way is gets pretty messed up and confused

Ok heres an example, i want to select B13 - B14 and apply the same formular that if B6>610 then chnage it to red,
then i want to select E13:L16 as a block and apply 2 seperate rules to say if B5>2440 then turn red and if B6 >1220 then turn to red.

can i do this together to save time or do i need to do each individually?

ok thanks the $ thing is exactly what i needed to know by the sound sof things,

the reason i want it to check B5 and B6 for those cells is because they are to represent the media going on to sheets of plastic, so if the length and/or wedth is over a certain size then i want a warning to flag up.

Cheers