First, Spinoza is not using infinite in its modern mathematical sense. For him, "infinite" means "lacking limits" (see Definition 2, Part I of Ethics). Second, Spinoza distinguished between "absolutely infinite" and "infinite in its kind" (see the Explication following Definition 6, Part I).

Something is "infinite in its kind" if it is not limited by anything "of the same nature". For example, if we fix a Euclidean line L, then any line segment s within L is not "infinite in its kind" because there are line segments on either side that limit the extent of s. Even a ray r within L is not "infinite in its kind", because there is another ray in L from which r is excluded. Among the subsets of L, only the entire line is "infinite in its kind".

However, the entire line is not "absolutely infinite" because there are regions of the plane from which it is excluded (although the limits are not placed by lines).