It can be difficult to get among plants to weed, water, feed and harvest without accidentally stepping on or damaging the plants if you grow in one large bed. A good width for beds is generally about four feet (1.2m). This size enables most people to reach into the center from each side without having to step on the soil, but this will of course depend on your own personal reach.
Growing in narrow beds like these also means that the soil doesn't get compacted since you never need to walk on it, which is better for the soil and your plants.
Paths between beds can be permanent, for instance using paving slabs or gravel, or grass will need to be mown regularly. Alternatively you can make your paths simply by leaving an area of soil untended, however this can become muddy and weedy.
Another popular option is to cover bare earth paths with wood chippings or similar to suppress weeds and make them cleaner to walk on. You'll need to replenish these path mulches at least annually.