Well, you mentioned bigger and thicker ;) They are not huge, more like low or mid medium in absolute sense. From really small things that are backdoor-dedicated and good (I don't really know anything about classic dildos), there are only two I can think of, but they are not carried by this outfit:
1. Obsession from Master Series
2. Red Ringer from the Doc's Red Boy line, often labelled as 'Red Boy Anal Wand' or some such
They are both seriously ringed, a lot smaller at the top than below (the ringer starts from 1 inch and goes up), and are good for training. Google for these, even if you confine yourself to this place.
Maybe try a few different butt plugs to warm up to, different shapes might be useful to think about as well as sizes. Wear butt plug for a while then try the dildo... With loads of lube. That usually helps me get things in lol! Keep relaxed and don't give up! :)
Unfortunatly it was too big :( could only get the tip in!
Perhaps rather than investing in new toys, you could try some muscle exercises, which will probably be more helpful. Have you tried consciously relaxing the sphincter muscle? It's not the same as the natural function speedup — try to achieve the same thing, but without the internal pushing. At the same time, keep gently shoving the toy inside, or at least keep it steady where it is, for a longer while, preferably. The key is to relax, as your muscles can take a lot and then some, quite naturally. It's all about harnessing that power in a conscious fashion, plus repetition of course once you manage something.