A planet that is entirely liquid

If such a body is constructed and maintained, yes, it is entirely possible - up to a certain size.

Under very high pressure (gigapascal range), water turns into one of several phases of ice, even at fairly high temperatures (see graph). For comparison, the centre of the Earth has a pressure of about 330 to 360 gigapascals.

Beyond the critical point at the right end of the graph, gaseous water cannot be liquified by pressure alone. Above the critical point there exists a state of matter that is continuously connected with (can be transformed without phase transition into) both the liquid and the gaseous state. It is called supercritical fluid.

Hence, if you keep temperatures between 0 and 350 Celsius throughout your body, and keep pressure below ca. 400 megapascal, you can have a fully liquid water planet. I don't think a few hundred km diameter will be a problem, but with more than about a thousand km diameter, the pressure will likely grow too high.

As the surface of the body is liquid, there will be some evaporation, forming a thin atmosphere of water vapor. Water vapor is an extremely efficient greenhouse gas, so the body should be carefully placed so that there is no runaway greenhouse effect, yet also no freezing of the water, even at the poles, which might be difficult to achieve. Also, if the atmospheric pressure drops below a certain (very low) level, liquid water cannot exist - it transitions directly between ice and vapor (see graph). If you count the atmosphere as part of your planetoid body, then an entirely liquid body hence cannot exist.