plate vs solid element

hello i have a question, 

i have seen that for plate elements, we have that the dimension should be greater than 10t and for solid elements that is should be less than 10t.

does that include the mesh size or is it the structure itself?? let us say i have a 20mx20mx1m slab and i want to model it. if my mesh size is 1mx1m should i use plate for that or should i use solid element? 

Thank you very much