There a lots of different ways to randomly distribute traps (also depending on trap size) for this purpose, which mostly work well. One important point to look at is which ecosystem you are working in. This should help to define trap size.

The plot size does not necessarily have to be the same. However, the number of traps you set in each plot should be equivalent to the surface area, i.e. you should have the same number of traps/surface area in all plots.

I would use a grid and randomly place traps (if traps were not circle or square I would also randomly orient them) in the plots.  Why do you need to have different sized plots?  I would do some pilot sampling first and use that data to determine sample size.  Have you seen this paper? http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.128.5581&rep=rep1&type=pdf

If you are going to compare the areas statistically, then it is better to have the same plat size. Size of the plat also depend of the habitat etc you are gouing to study e.g. for bigger plant, you need beger plat and for small plants you need smaller one.

randomization, make no difference, you can use table or right the name or number on paper etc, put them in a bag, mix them and select randomly from a bag to put in each place.

However, if you are going to publish your work, which you will do I think, it is better to use references from the litreature from those who have done similar work, because the refree will probably ask for standard method.

Good Luck

To use random numbers to sample seed rain, you will need some reference scheme to which refer them. The grid idea of John Dilustro is a very good one. However, you need to take in account the grid size compared to the trap size (I assume the first will be bigger than the second).

Regarding your secodn question, what needs to be equal is not the size of the plot areas (that normally depends on external biotic and abiotic factors), but the density of traps (i.e. number of traps per area unit: for example traps/ha).

Random placement won't work because you need to collect actively dispersed seeds deposited by birds and passively dispersed intact fruit under the canopies. So, obviously traps must be placed under perches and under branches with fruit. Remember, if a trap is not placed under a seed, it will not collect a seed! You should place transects in radial directions with increased numbers further from the tree in case seed deposition is anisotropic. See my paper 'Size-based fruit selection by a keystone avian frugivore and effects on seed viability', DOI: 10.1080/0028825X.2016.1247882, which describe seed-trapping in detail. Plot size depends on canopy area of trees – you must sample at least 6% of the area over which seeds are dispersed.

Andrew :-)

I agree with Luis, the density of traps is important, not plot size! Btw, the paper I mentioned is still in press, but I can send you details of my trapping method :-)