Why do the hormones FSH and LH decrease when the follicle is maturing and does size of follicle change during menstruation phase
I find FSH and LH hormones do actually
  decrease during day 4-14. Is that because of increase of
  oestrogen？


You first need to note that during one ovarian cycle, multiple follicles are stimulated to grow at the same time by FSH. However, one follicle ends up being the dominant follicle, and it is the oocyte housed within this follicle that is released on day 14. The dominant follicle is the one whose cells express gonadotropin receptors the earliest and so have the greatest sensitivity to FSH. Its growth would outcompete the other developing follicles. During days 4–14, FSH begins to decline because the granulosa cells of the follicles secrete inhibin, which—just like in males—causes the release of FSH and LH from the pituitary to drop. Only the dominant follicle is capable of withstanding this FSH drop; the other developing follicles are thus eliminated.


  Also, another question is will follicle
  grow during menstruation, because on diagram,the size of follicle
  actually changes during Day 0-4, and FSH and LH hormones increases.


As user19099 explains, yes, follicles grow during menstruation. With the degeneration of the corpus luteum, the negative feedback effect of estrogen and progesterone (in combination) on the hypothalamus is relieved. Thus, GnRH levels increase, causing FSH and LH levels to increase (as reflected on the graph you included), and FSH will stimulate follicular development.



References


Marieb, Elaine Nicpon, and Katja Hoehn. Anatomy &amp; Physiology. San Francisco, CA: Pearson/Benjamin Cummings, 2008. Print.