Is it your first psychedelic experience? I had taken LSD a couple of times before I did shrooms so I started with an eighth. If it is your first trip ever, I would start with a half eighth.
The come up of the eighth has a lot of closed eye visuals of very complex geometric patters, I could also see lyrics to some of my favorites songs floating in my mind. When I peaked on the eighth I was listening to music and content of the song shot me off to the world it was explaining. When I heard Yellow Submarine, I would see myself riding in a yellow submarine with the beatles. When the song mother by John Lennon came on I was shot into another world where I reunited with my mother who had been dead for 15 years. . The comedown was pure bliss. I loved everything and there was not a care in the world. It was wild. If something of that caliber interests you, go for the eighth.
Taken from Erowid.
Medium Effects: (0.75 grams - 2.5 grams P. cubensis)
"Cold" feeling, gas and/or stomach discomfort, nausea, pupil dilation, open eye visual effects: lights exhibit auras, star-pattern effects, rainbowing around lighting, lights seem brighter and often "more beautiful", notice movement in periphery, sometimes increased ability to focus, sometimes reduced ability to focus, visual field "distracting", visual field "entertaining", closed eye visuals: normal closed eye "blobs" take on patterns, shapes, distinct forms, increased ability to visualize creatively, spontaneous detailed images, feelings of time dilation, feelings of "coming home again" (often more pronounced for those who have used mushrooms before), feelings of belonging and connection, increased emotional sensitivity, increased ability to focus on emotional problems or memories, chance of thoughts becoming "caught in a loop" / dwelling on a single thought or feeling (usually negative or painful), realizations about past feelings, realizations about how to live, gain a new perspective on current lifestyle and behaviors, feelings of connection with others, noticing things that are normally ignored or taken for granted, feelings of wonder, spirit, joy, sadness, despair, religious awakening, contentment and possibly latent psychological crises can come out.
High Dose Effects: (2.5 grams - 10 grams P. cubensis)
All of the Medium Effects, usually with a significantly more uncomfortable coming up, more pronounced nausea (rarely resulting in vomiting), sometimes significant mental discomfort associated with feelings of fear and oftentimes accompanying a "What have I done to myself?" or "How far am I going?" thought, the unpleasant coming up effects usually lessen with familiarity and more knowledge about the safety and character of mushroom effects. High Dose Effects are usually characterized by the closed eye visualizations being significantly more elaborate and enfolding, religious revelation, spiritual awakening, near-death experiences, loss of self, talking to seemingly external, autonomous entities, extreme emotional responses, repressed memories coming to life, latent psychological crises can come to the surface, an increase in artistic sense, and intense feelings of wonder, connection, joy, fear. High Dose Effects can also include extreme time dilation, with experiences of wall-clock minutes taking an experientially large amount of time, watching clocks or digital counters where the seconds seem to take minutes to count off. One of the most interesting effects is the feeling of awakening for the first time ever from a previous state of sleep, of liberation from what is now seen as a life-long state of bondage. Paradoxically, it is this new awareness that feels normal and natural and the previous fog is seen to have been unreal all along. The bemushroomed seeker is convinced that, once gained, this awareness is impossible to lose, but inexplicably by the next day it is just a memory.