
About practicing visualizing a deity, or yidam, I have read in a dharma book (I can't remember in which book) that for beginners
using a rather small picture, or small statue is advisable, as well as always following the same given sequence (in the sadhana) of
the to be visualized parts of the Buddha on the picture, avoiding going into details, and doing the the visualization rather quickly.
I am happy to have discovered that a small picture/statue is best to start with.
I was using a poster size print, thinking 'the bigger the better'.
Funny thing is, since I have put a simple picture of the by me to be visualized Amitabha Buddha as screen saver on my mobile phone,
I notice that I can easier visualize Amitabha Buddha now.
Throughout the day my mobile might stop me in my tracks five times or more (incoming texts), and now offers the opportunity to quickly study, in the proscribed sequence, the bright small picture for maybe 5 to10 seconds, until the light goes off.
Sometimes, when walking around the house and my eye catches the mobile on the table, I might stop, do 'an imprint action',
and carry on with whatever.
All these small doses of imprints have helped me to visualize the outline, lotus throne, color deep red, hair and head, eyes, mouth,
bowl in hands, of Amitabha Buddha more clearly, and little by little I am adding more details.
Wherever I close my eyes a more detailed image of Amitabha Buddha than ever before, various colors included, appears in my
mind's eye. (I am using a mobile-phone since a year).
What helps me to stay focused when gazing for that short period of time at the picture, is to fast softly recite my mantra, and quickly imbue the two dimensional colorful picture with (my as yet feeble) calm, unconditional loving kindness; sometimes it feels a bit like merging with that aspect of Buddha Nature there and then, because somehow the picture seems to become 'alive'.

I do understand now a days that to help keeping focused, rapid but clear recitations heard by yourself, and swift well focused visualizations are better than slow ones.
I also read that when sitting/standing in front of a big statue/picture of a Buddha loud recitations are appropriate,
and when being in front of a small statue/picture of a Buddha softly spoken recitations are appropriate.