Is there a better word or phrase for “somewhat ostentatious”?

Perhaps you could use excessive. The meaning could be interpreted both literally (size) and figuratively (intention) depending on how you craft your supporting statements.

I think the even more simplistic 'too much' or 'overdoing it' would work. As in,

Don't you think a two page ad is a little too much?

or

Don't you think a two page ad is overdoing it?

If you really want to put in the nuance that it is somewhat narcissistic/egotistical/ostentatious/self-aggrandizing, you could say:

If we're going for a two page ad, we might as well skip that and have them name a building after us.

I often favor the word overwrought when I'm trying to be as diplomatic as possible about ostentatious language. Unfortunately, it is an uncommon word and your using it, ironically, might trigger the reaction you're trying to avoid if it seems that you are trying to speak "above" them. If you think they are comfortable with that level of vocabulary, you might use "overwrought." As in, "Don't you think the ad a bit...what's that word...overwrought?"

overwrought (comparative more overwrought, superlative most overwrought, adj.)
1. in a state of excessive nervousness, excitement, or anger; Extremely tense, anxious, or upset; filled with emotion, emotional; uneasy
2. elaborate; overdone

Gauche, as given by Tom above, is also a good alternative.

I would say somewhat ostentatious is pretty diplomatic,

Gauche might be an alternative, although that implies a lacking of social grace which may or may not be what you mean.