As a linguist who reads laws for a hobby, I would say that "and" legally means what it was intended to mean. There are often interpretive statutes which say that "and" can be read as "or" or vice versa, when necessary (as in ORC 1.02
"And" may be read "or," and "or" may be read "and" if the sense requires it.
Delaware doesn't have that as a rule, but it is a rule employed by courts "as required".
One approach to interpretation is to discern intent from surrounding text, so we would look at the whole code. The general context is the rule that "A building or land shall be used only for the following purposes". Following Article XXVII of the code, other uses could be permitted because "§115-32. Special use exceptions may be permitted by the Board of Adjustment and in accordance with the provisions of Article XXVII of this chapter and may include...". That section ends with "C:Other special use exceptions as follows", and includes "Private garages for more than four automobiles and with floor area of more than 900 square feet in a residential district". From the list of things enumerated in §115-32, there is no coherent pattern – some things are in the list of special exceptions, some things are in this list, some not. So the "surrounding text" approach doesn't help in this case.
Scrutiny of legislative debate is sometimes invoked, especially at the federal level, but there is negligible chance that there is any such evidence here. The almost-final approach is to spell out the competing interpretations, and see if anything jumps out as ridiculous (because it is assumed that lawmakers do not pass ridiculous laws). The two interpretations are "both must be true", versus "one must be true". Since the general rule is that you can go ahead unless it is restricted, then with the "both" interpretation, you need a special exception permit if you simultaneously plan to have more than 4 automobiles (which means, 5+, so 4 is allowed) and floor area greater than 900 sf. Thus if you plan for only 4 cars, or can fit the 5 cars into 900 sf, then you would not require a permit (on the "both" interpretation). Which btw is the literal interpretation of "and". This is not an absurd scenario (using a generous 10'x18' space, which I derived from parking slot regulations in Danbury CT). So it is reasonable to think they meant "both".
The "either of these" interpretation says that they are being even more restrictive – you need permission to have a 5+ car garage (regardless of size), and you need permission to have a garage larger than 800 sf (even if there were only 1 car in it). This seems a bit specific since there isn't generally a size restriction on structures in the code – except that playhouses are limited to 150 sf. and can't be tall enough for an adult to stand up.
Since the literal meaning of "and" is "both at once", and since no facts about the code say otherwise (i.e. that interpretation does not result in an absurd nullification of some other provision), an objective court should interpret this rule to mean "both at once", thus the government imposes the fewest restrictions on your property. No way to know what they will do.