Your question is very broad one so i'll try to answer it briefly in sections.

1. First of all SPR is directly proportional to the number of nanoparticles produced hence the absorbance of your sample will increase as the nanoparticles are generated within the reaction mixture. Change in the colour is a primary indication of nanoparticle production.

2. An increase in reaction time results in increased production of nanoparticles hence increasing the reaction time will result in increased absorbance measurement using UV-vis Spectrophotometer.

3. As for temperature, its effect is highly dependent upon the nature of reducing/capping agents found in the plant extracts so you should focus on finding the optimal temperature for your own extracts. In general an increase in temperature would result in an increase in absorbance as increased temperature favours nucleation process.

4. Size and stability of nanoparticles again depends upon the nature of reducing/ capping agents. Since exact mechanism of capping and reduction is not yet known, its hard to tell the result before you have performed the experiment. In my personal experience, biosynthetic reactions that occurs vigorously shows less stability and formation of large sized precipitates that settles down at the bottom with the passage of times while reactions that occurs slowly are more stable.

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