Your best bet is to get a globe rather than a flat map because the globe will give you a true feeling of size ratios between landmasses. A standard flat map, usually known as a Mercator Projection, actually makes land masses which wrap around a sphere fit on to a flat plane by fudging their actual dimensions. An alternative, actually a compromise, with a flat map is to get get what is known as an Asmuthal Projection. You have to get it with its center based at the grid coordinates of the middle of the area of interest. The landmasses do NOT look like you remember them from flat maps in school, etc, but will give you an accurate comparison between different landmass sizes, with a lot of surprises along the way! I am sure there are many websites which would offer Asmuthal Projections based on you needs, but Im sorry I just cant direct you to one
MarkS