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I am a visual artist who predominately creates realistic acrylic paintings of the human face and body. As described above, my boundaries would have to be a nice amalgam of thick and thin.
When interacting with others I am vagrantly self conscious about boundaries and take care not to cross them. In return, I have an aversion to even impersonal touches when communicating with others. I do not like to hug, hold hands, touch arms, or stand too near to anyone with whom I'm not intimately personal. I am obsessively organized and time oriented. Every day at 10 o'clock a.m. I take my break. I travel the same route to work every morning, although there are others that are just as quick and efficient at getting me to point B. My desk would make Monk proud. I enjoy realism over impressionism, physical boundaries in my home (and themes for each room), and I'm an introvert who does not like socializing or being in a group of people--All because I have thick boundaries.
On the other hand, I experience deja vu often, I frequently mistake my dreams for memories and vice versa, I have identity issues including with sexuality, I fall in and out of love easily. I'm a daydreamer. And, as I mentioned, I'm an artist.
The answers to your questions as I see it:
Optimal thickness: Subjective. People's boundaries start forming in early childhood when self is identified and are reinforced by personal experiences throughout life. I believe this density is fluid over time and very variable person to person. (In short, there are so many degrees to thick and thin boundaries that clumping persons into one mass category either way would be devastatingly misleading.)
Creativity and boundaries: Personally, art inspires an open mindedness other disciplines may disapprove. Thin personal boundaries may be the intuitive response of some artists. However, many artists are much like me, I would wager--having difficulty with interpersonal relationships because of an inability to extend their boundaries beyond surface level. In other words, at first glance these thin boundaries appear to be an open gate of self and other intermingling. On closer examination, we may begin to notice that the open artist is an anomaly. The artists I know are quite often very conscientious of who gets in and who stays out. Maybe we have no set boundaries. (Fluidity again?)
Control over boundaries: My gosh, yes! And, no. WE have control over our boundaries to the extent of allowing others to be close or stay their distance, physically and psychically. We have control over how rigid we are in allotting the boundaries. We may not have had control over the events that formed thick boundaries. We may not have had control over the events that fostered thin boundaries. This lack of control in past experiences may encourage a sense of lack of control in our future personal interactions. However, the moment you decide that you want to break a few rules you've set for yourself or set a few rules you've routinely broken about your interaction with others, you begin to redefine your boundaries. This is a process that happens continually, even daily.
The artist in me still rebells at coloring in the lines anyway. ;)
