Help doing conditional formatting based on size cell to colour other cells

Hi I'm creating a complex cost calculator for a company with a table of base costs/mark-ups, etc which then calculates a separate set of prices for the customers based on sizes entered, so far its going ok, but I'm very rusty with excel so i have a few sticking points i need help with.

firstly i have 2 boxes where length/width are entered, then a table of all available products which the costs update based on the sizes entered, thats working fine so far.

what i would like to do though is program those cells to highlight red if the size entered in the length and/or width are greater than a certain value (specific to different media based on their maximum size) so it would serve as a warning that the price shown is correct but that it is larger than what is possible given the maximum size.

so I'm thinking i need to highlight the cell i want to have the warning colour and apply a new rule which says if the length >1200 then colour the cell red...

the maximum size is different for different materials so i don't want the length/width cells to have the warning,it needs to change specifically other cells based on their maximum size.

how do i do this? can't figure it out.