importing stills malarky

Hi,
forgive me, I have not edited anything for years and it's all too new.
I am importing a lot of stills into FCP X for a project shot on an old PD150 ( PAL).
Some stills are very high res and I am thinking it would be better to fix them in photoshop first so they don't clog up too much space.
Trouble is I don't know what I am doing.
At all.
I just have Photoshop elements, so there is no option for converting to PNG as I have read is suggested, so am sticking to Jpeg.
I am documenting a period in time after a natural disaster so am relying on other peoples photos- which will be of all kinds of resolutions and qualities. At this stage I am just winging it, teaching myself FCPX as I go.

My questions for all you awesome folk out there are-
Do all my stills have to be the same size?
Is 200 dpi a good resolution to resize to?
What about cutting down the physical size- can it be like 20cm by 15cm or 60x40cm or must it be higher to look good? Or can it be random? Or does this depend on my final display size? ( a big flat screen telly in gallery setting )

I am sure I can sense you rolling your eyes- please forgive me, my talents lie elsewhere ;)

One other question, I have done quite a few dissolves on my stills sequence already and they look utter shite! Not smooth. elegant or subtle in any way shape or form. Not that keen on fade up/down from black either.
Is this FCPX or is it me?

Thanks so much in advance,
Heidi

'Getting back into making after a long time off'