I can take female fictional characters seriously with boobs of various shapes... but as far as sizes are concerned... I process sizes below 'huge' on a more serious note. It also depends on how they're portrayed in whatever particular manga or anime. Boobs have a tendency to ruin series for me though. Some kinds of fan service (involving mostly boobs)... have a tendency to bring my interests to a screeching halt. For example... I have a sexy pin-up tattoo of Neliel Tu from BLEACH... but I nolonger watch the series... and I collect the manga very sparingly and out of chronological order for particular art references. The exception to this rule is the Witchblade anime. I have a bikini pin-up tattoo of Shiori Tsuzuki... modified with tattoos of her own and extra added curves. I have collected the first box set of the Witchblade anime... along with the american soundtrack. Boob anime has a tendency to make my mind go fuzzy... and Manyuu Hikenchou is a prime example... entertaining... but the boobs fry my brain. Mouse and Eiken are f&^%ing terrible... but would probably be tolerable if I was involved in the creative process.
I really don't mind, I'll take on anyone and any size, so I'd say past huge, just not GARGATUALLY huge if it's not comedy. I got used to this kind of thing when I saw Cattleya from Queen's Blade. DAMN those things are HUGE (probably the biggest I've seen in an anime or manga that's not hentai), and she's a really good fighter BTW. Others like Tomo (Sekion no Qwaser) Manyuu (Manyuu Hickenchou) and almost the ENTIRE Eiken cast just use it for comedy, though MH takes it a bit more seriously, to the point where it's PLOT IMPORTANT.
I'll say huge; it makes a bit of sense to me. I do see and handle a multitude of sizes, but I think huge is about as big as as they should get if you want a character to be able to have a serious tone to them.
Had to refer back to the scale to remember how it sits, but I'd say probably Large. Before, I could have probably accepted much larger sizes (Big and/or Huge) and still taken it seriously, but it's gotten to where I've seen so much over-sizing over the past year or two in anime and the like that my tolerance has lessened. My comfort zone has become Normal-Large due to thinking more realistically than I used to.
Not that I won't stop looking at the Huge and beyond images. It's just if I want to take something seriously, it should be realistic in multiple ways.
I'd say "huge" usually, but it is more complex than that.
Some people here said it depends on the actions or outfit, but I'd say it's the way you draw. I don't mean to offend, but I'm thrown off the mood and atmosphere of some of your comics when something really dramatic is happening and at the end the character is with arms crossed, breasts falling over them. They just pop out too much.
You're really good at "breast physics", Matt, but they are usually stylized to fit the funny mood. More serious scenes would call of more realistic physics, clothes and fabric not so skin-tight, etc. That applies to "elegant" oufits that you mentioned before.
I think what I'm trying to say is that it's more about the way the boobs behave than the size. If they aren't the focus the character can shine.
Bit-to-huge is OK for me, but I really enjoy learning how unfeasibly-large-breasted girls manage day-to-day. That is, if their characters are realistic and three-dimensional, in their own worlds, then the size their body parts is not so important.