Wormholes

C1 unlikely anyone will bother you so just depends how disposable you wanna go, C2 with null/C5 you might get some interest from C5 residents. General advice is to go large and fully ecm it up below C5/6 space if you intend to be somewhat resident otherwise doesn't make much odds.

wont a medium one do with that??since it will stay reinforced for a long time and you can still mount 10 ecm's for each sensor strenght.

Best is to have Large POS everywhere. If you are having problems feeding pos in wormhole you are doing it wrong. C2s are swarmed by PvPers. Large is a must there. But from my experience i lived about 3 months from medium pos in C4 without any problem. I met a guy who lives out of small pos. But its C4 space, Carebear haven.

You can siege a small tower in about an hour with a few Oracles.

Go large.

Large Amarr POS with 4 neuts 2 webs 2 points 6 guns 4 Damps 1 SMA 1 PHA at least 50% omniresi is all you need to chase of most fleets, add ecm to free cpu/grid, its a very basic setup able to defend itself against a Fleet with only 2-5 Guardians ;)

However a ECM annoy-star is the way to go for below c5/c6 to annoy people at most possible levels

Keep in mind that C1s doesn't allow to to get battleships through, so if you're using that to rat I think that C3s are best. This is because a C2s with null sec statics also comes with a C5 static with an identical signature strength (which makes it a 50/50 chance to open the C5 hole instead of the NS hole when you going out to rat).
https://wiki.eveonline.com/en/wiki/Wormhole_Signature_Strength_List
This could mean that you open up a hole into a C5 PvP corp system, which in turn could mean that you'd have to risk popping it, or play hide-and-seek with them until it closes.
The C3 would provide you with better sleeper NPCs to grind aswell.
So, all in all:
C1s - Safest due to the general uninterest and Large towers being a pita to seige in there. A no-go if you need to use Battleships to rat. Rolling holes in here isn't worth it, so that could make it a no-go aswell. Doesn't really offer that much except for the static and calamity.
C2 - Allows for Battleships to move around unhindered, but the high amount of traffic (lots of holes( and both statics being on the same sig strength, I'd avoid these unless I had a special interest for C5s. The gas maybe?
C3 - More interesting than the C1s for the outsiders, but less so than the C2s. Only one static, so no worries there. C3 anoms pay fairly well, so these add to the worth of using a C3 system.

Note:
Wandering holes provide both dangers and benefits in all 3 systems. The danger with these is that they can be dangerous to deal with unless you are careful. (As in making sure that the wormhole you just scanned really is the hole that you want to open up). And as for benefits, the wandering wormholes also provides you with an unregular routes to HS/LS, which is excellent/decent for logistical purposes.

As for more information, I'd suggest you to read the guide included in the stickies on this forum.

Large guns are all you need.

Your POS defenses, whether you go large, med, or small, are all about mathematics. Some guy, a guy less drunk than I am usually, will look at your POS. He will plug it into a spreadsheet (this is EveE, there is a spreadsheet for it), and decide what it will take to crush it. If he has that much "what" around and feels like doing it, he will crush it. The idea behind POS defense is to make the "what" high enough that it isn't worth the trouble. Baring that, it is up to you to bring more defense than he brought "what" when the reinforcement timer is up.

If you have no enemies and don't show off a lavish lifestyle by leaving ships out of the SMA that are shiny, then most people won't have the wherewithal to load up the spreadsheet to begin with.

My advice is to go large if you intend on staying in the hole for any length of time. Put a mix of hardners, guns, ecm, and scrams/points on it. Haul out your loot at every opportunity. Only keep the ships you need for your goal of "ratting in nullsec" (which sounds horrible to me, but I'm trying not to judge). If you only intend on carebearing it up, eventually someone will get mad and bring a ton of "what" to your hole and burn you out.