We have both, and I'd tend to pronounce "Wirth" similar to "wit" as far as the "i" goes. It's not always clear just from looking at the letter. But some words have explicit cues: There are "stretching consonants" like a-a, a-h, e-e, e-h, i-e, i-h, etc: Aal, Kahn, dehnen, dienen, sühnen, etc. And sometimes the following consonant gets doubled up to indicate a shorter pronunciation, like in "Bitte".
