Hi,
opening the aperture is almost a requirement for that kind of photography, but I suspect your camera did it already. You normally adjust the aperture via the A or Av mode. A smaller number means a wider aperture. Lenses generally do not perform at their best wide open, but it's often less visible on a P&S.
Regarding the size of a DSLR, only you can decide what size suits you. Manufacturers always offer small entry-level models, and cameras from Pentax, Canon and especially Olympus are quite small (though stilllarger than a compact, of course). Panasonic also launched a "micro four thirds" camera that's somewhat expensive but even smaller. Prices also have come down, but again it's a very personal decision.
A DSLR will always give you better images, but the difference is not always visible. Only you can decide what camera is best. Remember that not many cameras nowadays are "bad" so there is no really wrong choice
