You can use either "a one-hundred" or "a hundred". I am not aware of any prescribed grammar in this circumstance. Maybe because it has become an all too familiar usage.
But, just to be analytical: "a—used as a function word before singular nouns when the referent is unspecified and before number collectives and some numbers "
Technically, 'a' in this circumstance is an indefinite article (singular). Which means it can be any 'hundred', not a specific 'hundred.'
Where these examples refer to a specific hundred:
1.) This hundred bed hospital.
2.) That hundred bed hospital.
3.) Your hundred bed hospital.
'One-hundred bed hospital' itself does not refer to a specific or non-specific one-hundred bed hospital. It only really creates an lexicon image.
One is not required before hundred because in this case (and most all cases) hundred is a noun. Just the same as any other noun, it is understood already as '1' singular. (Cat or One Cat, bed or one bed)